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hello everyone i'm saying hey dari
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and
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i'm going to be your instructor these
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couple of lectures on polymers
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we started our discussion
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in the last lecture about polymers we
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introduced some concepts in terms of
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the definitions
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what a polymer is made of
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how these
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structural features and the chemical
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composition of a polymer can influence
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its mechanical and chemical properties
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and we
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brought up a few different
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polymers that are frequently used in
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industry
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and so
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you kind of got an idea of the breadth
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of things you can do with polymers in
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terms of applications but this is just
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the tip of the iceberg there's lots to
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discover
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and hopefully in the next few lectures
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we will talk about different
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aspects of
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polymer chemistry
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we've talked a little bit about
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production of polymers and how we
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synthesize these
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large macromolecules
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and we will talk more about that as well
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as
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basically
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some main structural features that
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determine
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whether the polymer
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shows a particular behavior or not
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and whether it's suitable for your
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application or not
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so
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uh i'm going to pick up where we left
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in the last session
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uh we talked briefly about how
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we synthesize these long
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molecules
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and one such method
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is free radical polymerization
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so in free radical polymerization as we
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uh discussed
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you have a free radical
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molecule
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that
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starts a cascade of reactions
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by breaking or opening that double bond
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in a monomer
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and forcing it to become a radical
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speaker itself so what it does is that
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it sticks to one end of that monomer and
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then breaks that bond
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the other end the carbon is now going to
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have one extra electron so it's ready to
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form a new bond and so it acts as a
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radical itself
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and this chain reaction continues and
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more and more monomers are added to this
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main backbone chain
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and the reaction continues
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until it's terminated
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now
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this reaction could be terminated in
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many ways but the main point is that
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you have no control over
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what the length of these molecules are
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so you can have a distribution of
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chain lengths
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all the way from very small molecules to
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larger ones and in this production
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synthesis cycle there is no way to
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control
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having 100
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of
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um you know x number of
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monomers in terms of length you will
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always end up having a mixture
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and what you can do
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is you can then filter these
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out and basically extract the subsets
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based on their size or weight etc etc
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an example of
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this radical that
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bond
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is
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benzoyl peroxide is an initiator what it
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does is that you have a molecule you
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break it somehow
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into two pieces two radicals
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and this is
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that um initial step in the process
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where this is the molecule that starts
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this this cast so
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how can you break that bond there are
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multiple ways of doing that as well
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one such method that we use particularly
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in my research is
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using light so when you shine light when
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you expose these molecules
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amongst many other
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ways to transfer the energy you can
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actually break this bond and in other
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words cleave this molecule into two
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radicals which actually starts this
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reaction so that's why it's a very
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interesting way to 3d print objects is
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using light and the way it works is just
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as i explained
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so
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based on what i just said you always end
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up after the polymerization reaction
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having distribution of different um
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polymer molecule lengths so you can have
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all the way from few monomers to
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you know
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so in this case you have a distribution
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plot that shows you actually um how many
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of your
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molecules fit in this
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molecular weight
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interval
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and how many fit in this interval so
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looking at this you can tell that the
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majority
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of the polymers that you got after the
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synthesis
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fall in this 20 to 25
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times 10 to 3 grams mole of molecular
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weight similarly you could have this for
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the number of monuments like for
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instance you would have they're like
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maybe the majority of these are 100
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monomers
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in length
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we will talk about that also in a bit
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but the main takeaway point here was to
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remember that you can draw this
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distribution based on the number
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of molecules fitting in each of these
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intervals which is the number fraction
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or based on the weight fraction which is
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how much of the weight of this entire
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sample that you've got
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is
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polymers with 20 to 25
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times 10 to three grams more molecular
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weight
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how
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much of that weight is polymers with
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another
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molecular weight so you can distribute
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these based on weight fraction which is
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the percent weight consumed by that
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group
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or the number fraction which is much
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simpler the number of molecules
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belonging to that
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particular group
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so an example for that that we had here
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i'm going to get to that in a minute um
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but
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the way you you then calculate this is
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that if you want to find the average
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um it can be either a number average or
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a weight average based on whether you
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use your
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your weight fractions or your number
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fractions
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and this was an interesting observation
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i still haven't explained it um i'll
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leave that to you at this y it's more
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frequent that we see the weight average
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larger than the number average but
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before that um let's see what what you
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use to calculate
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these two
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if you want to calculate the number
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average molecular weight you you find
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the average based on these number
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fractions so you sum up
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all the number fraction
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molecular weight products in that table
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or in that
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plot that you saw there
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and that will give you the number
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average if you want to calculate the
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weight average you find the product of
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the weight fraction
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and the molecular weight belonging to
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that
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particular column and then you
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sum up all these products to find the
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weight average molecular weight another
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way to do that is a lot easier is if you
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had the overall weight of your polymer
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sample
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and you also knew how many molecules you
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had
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then you can easily just divide those
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and that will give you the average
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molecular
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weight so that is again a number average
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but it depends on what data you've been
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given
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whether you can use
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this formula or the other one
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so
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um
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we will have more examples on how to
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calculate this in class and the quiz and
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also we'll solve a lot
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of these problems in class but
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for now uh i've actually brought an
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example that isn't
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um
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from
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this concept is a lot broader it's a
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concept in statistics that people use
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quite a lot is either the number average
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or the weight average for any quantity
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and in this example it's the weight of
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students in the classroom and
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what you've got here is
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in one column we've got the number
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fractions in the other one we've got the
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weight fraction
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and the way we calculated these
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was that for instance if i want the
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number fraction
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for this particular weight
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i have to divide the number of students
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with that weight
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by the overall number of students so
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that you've got 0.2
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or 20 percent of our class
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in terms of a number percentage has a
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weight of 220. now if i were to
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calculate the weight fraction i would
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divide
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and the weight consumed by this
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group which in this case we've got two
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students
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each have 220 so that's 440.
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i divide that by the overall weight of
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the entire class so i have to sum up all
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the students weights and it will give me
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1860 pounds and then i divide that 440
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which is the overall mass or weight
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of
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this
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this class of students which is like 220
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pounds
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by the the total weight and that gives
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me the 0.237 which is the weight
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fraction belonging to this
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class or category
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of weight
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so
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once you've calculated these fractions
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now you can calculate the
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um the average mass and the way you do
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that is you multiply the fraction by the
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mass so you've got the products of these
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two columns and you sum it up and you
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get number average similarly you find
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the product of the weight fraction and
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the weight
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now respectively and you sum up all of
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these products and you get the weight
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average mass so it's a pretty simple
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process but just make sure that you
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don't make
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excuse me a mistake in calculating your
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fractions because then that would
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influence your overall result
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so there's another metric too which is
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degree of polymerization so degree of
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polymerization is the number of monomers
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in a chain which is a lot simpler than
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molecular weight
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this is just simply how many monomers
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connected together to form this chain
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and in this case for instance uh our
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monomer has two carbons so this is a
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polymer
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that consists of six monomer units so
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you've got
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a degree polymerization of six so
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it's interesting you have similar to
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molecular weight you can find a number
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average
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polymerization and you can have a weight
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average degree of polymerization so
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again to use each of these you have to
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find the number fraction or the weight
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fraction
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of each
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molecule so for instance if you had a
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distribution
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this time instead of molecular weight
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for
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basically
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formulation you would be able to use
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this formula and calculate each of these
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products
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and sum it up
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so this is um again
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pretty simple it follows the same
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concept it's the same formula but rather
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than finding the average for molecular
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weight here we're finding it for degree
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of polymerization now degree of
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polymerization is really important
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because it tells us how far you've gone
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in terms of your
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reaction okay so
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if you have a degree of polymerization
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that really small that means that you
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either didn't give the reaction too much
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time so all the molecules were
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terminated pretty soon
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or maybe you didn't have enough energy
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in that reaction the temperature was low
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or it could have been you didn't have
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enough of the initiator
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if you have a
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larger
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degree of polymerization what that means
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like if you have 100 200
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even a thousand monomers in that chain
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what that means is that you gave it
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enough time or there was enough energy
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the temperature was higher there could
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be a lot of factors that influence that
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but you ended up getting a rather long
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molecule
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so
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now that we've defined number average
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and weight average um degradation
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uh we're going to talk a little bit
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about molecular shape
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so this is a distinct part of this
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lecture series we're going to look at
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how um the confirmation on
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configurations of these mod
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what we mean by confirmation
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configuration
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and how these can influence the
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properties of your
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so
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you're probably familiar with already
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molecules um
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are made up of bonds they could be
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single double or triple bonds and we've
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come across that earlier as well
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as well as atoms that are connected by
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these bonds
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and
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these molecules can be reoriented in
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space so you can actually rotate them
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but there are some things you cannot
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change
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without needing to change the chemical
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structure
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and basically break some bonds so you
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can easily rotate this atom
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as you can see here
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about this cone here
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and about this
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other bond that you see
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but you can't change this 109 degree
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angle
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this is a fixed amount coming from the
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molecular and atomic equilibrium state
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and all the force fields that are
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holding these atoms together so you
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cannot
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change this angle you cannot change the
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length of this bond but you can actually
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have this atom rotate about this point
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without needing to break a bond
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so just based on this simple
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philosophy
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we can actually
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understand how polymers these long
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chains of molecules can change their
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orientation and shape
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without changing their actual identity
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so
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for example here we have a chain of
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monomers you've got a polymeric chain
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here
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and you can imagine how each of these
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atoms can freely rotate there
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and therefore
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get into these
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equivalent
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since the molecule is still the same
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they're just reorienting themselves in
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space
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now it's really important to remember
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that
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it's not always as easy as they can just
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rotate however they want to
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depending on what you have in these side
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groups and depending on the strength of
00:16:02
this bond
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it may force
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the next atoms in the chain to stay in a
00:16:08
particular
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orientation or configuration
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so that's why
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um
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you can do some stuff but you can't do
00:16:18
others so
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and just to make it clear
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some changes
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are not achieved by just simply rotating
00:16:27
these for instance if you rotate this
00:16:29
molecule that you see here however much
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that you want to you won't get to this
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molecule
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because this one here is a mirror
00:16:37
of that molecule so if you keep rotating
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this
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you're never going to get to
00:16:43
this shape here you can try it yourself
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so
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when when you're given two different
00:16:48
molecules make sure that you don't
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by mistake
00:16:52
assume that this is just a reoriented
00:16:55
version of that
00:16:57
because in a lot of cases you mirror the
00:16:59
molecule and that's an entirely
00:17:01
different transformation
00:17:03
now
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what i was talking about earlier was
00:17:06
that even this simple rotation may
00:17:09
sometimes be
00:17:10
limited and constrained now
00:17:13
this is where you actually
00:17:17
talk about configuration versus
00:17:19
confirmation so
00:17:20
there are in some cases
00:17:23
situations where you cannot simply
00:17:26
reorient your molecule so if you have a
00:17:28
double bond for instance
00:17:31
if you have a double bond that double
00:17:33
bond
00:17:34
limits the movement
00:17:37
here so
00:17:39
similarly if you have a large side group
00:17:42
that also imposes constraints on the
00:17:45
next atmosphere because of its huge
00:17:48
force field that it's producing around
00:17:50
it and these are basically atoms um
00:17:54
rebel each other like magnets do so
00:17:57
if you've got this present here the
00:17:59
other atoms
00:18:01
are going to be constrained to degree
00:18:02
more than they were if this was just a
00:18:05
hydrogen so you can imagine that
00:18:07
some certain force fields like this
00:18:09
double bond or that side group can
00:18:12
actually put limitations on how much
00:18:14
rotation you can get now
00:18:18
for instance if you want to get this r
00:18:20
group there
00:18:22
over there you need to break that bond
00:18:25
into a single bond
00:18:27
then reorient it
00:18:29
and then
00:18:30
bring it back to a double point so what
00:18:32
this means is that you have to break
00:18:34
that double bond
00:18:36
so that it's now
00:18:38
weak enough for this rotation to happen
00:18:41
and then
00:18:42
bring it back to the double point state
00:18:45
you cannot just simply do that
00:18:48
when it was a double bond
00:18:50
similarly here you could actually change
00:18:53
this configuration
00:18:56
in your synthesis process you can
00:18:58
determine that but you cannot change
00:19:00
that afterwards if this was a double
00:19:03
bond in this case actually for
00:19:05
polystyrene you have indeed a single
00:19:08
bond so it can rotate
00:19:11
but because you have this large side
00:19:14
group it limits the rotation of the
00:19:17
monomers next to it so
00:19:20
these are two examples of what can limit
00:19:23
your rotation but other than that there
00:19:26
are lots of other cases too that's why
00:19:28
this is just an introductory module
00:19:31
towards polymer engineering if you
00:19:33
really want to understand why a specific
00:19:36
polymer behaves in a specific way
00:19:38
you really need to look into the
00:19:40
structure and look at all these
00:19:43
exceptional cases that may happen
00:19:46
so
00:19:49
knowing that we're going to talk about
00:19:52
uh isomerism and what isomers are is
00:19:55
that
00:19:57
they are molecules or compounds with the
00:20:00
same formula
00:20:02
but different geometries or different
00:20:04
structures
00:20:06
so
00:20:07
for instance here you have octane it has
00:20:10
eight carbon atoms and 18 hydrogens okay
00:20:15
so it follows the same formula that we
00:20:17
discussed before
00:20:19
and what it has here is
00:20:22
that this is a linear chain
00:20:24
you can reconfigure that
00:20:27
into a situation where you've got iso
00:20:30
octane and instead it has some branches
00:20:33
here so you still have the same number
00:20:35
of carbons and hydrogens
00:20:37
but here rather than having a linear
00:20:40
chain you have a chain that has these
00:20:42
branches coming out of it
00:20:44
perpendicularly so
00:20:47
these two both share the same
00:20:50
chemical
00:20:51
composition the same formula but they
00:20:54
actually have two different
00:20:56
structures two different geometries so
00:21:00
these two are called isomers
00:21:02
uh and
00:21:04
i just want to do a classification
00:21:05
before we look further into these items
00:21:08
isomers can be
00:21:10
stereoisomers or geometrical isomers and
00:21:12
this is what i'm going to talk about in
00:21:14
the next few slides is how these two
00:21:16
differ
00:21:18
and each of these have their own sub
00:21:20
categories which we will
00:21:21
briefly
00:21:22
so first let's talk about
00:21:25
stereoisomers now what are these um
00:21:28
basically these are items where you you
00:21:31
have a long chain
00:21:33
and these
00:21:35
side groups can actually be
00:21:38
shifted around so it's it's basically an
00:21:40
angular reconfiguration
00:21:42
but again as i told you
00:21:44
because these side groups can be
00:21:46
relatively large you can't just simply
00:21:48
go from this state pads by rotating it
00:21:51
so it's kind of locked into this state
00:21:54
that you see here
00:21:55
and what you see here is that you've got
00:21:58
an isotactic so we define tacticity as
00:22:01
the stereo regularity of the chain or in
00:22:04
other words how these
00:22:06
sequences are repeated so in this case
00:22:09
all of these side groups are on the same
00:22:12
side it's isotactic
00:22:14
in this case they alternate so you've
00:22:18
got one on each side
00:22:20
and in this case it's completely random
00:22:22
so it's a tactic
00:22:27
so
00:22:28
these isomers we call them stereoisomers
00:22:32
basically have the same chain structure
00:22:34
but the side groups are rotated and
00:22:37
shifted in and out
00:22:39
now this could be
00:22:42
completely different it could be that
00:22:45
it's this is just one very specific type
00:22:48
of isomer what i showed you in the
00:22:50
previous page the previous slide
00:22:53
was that you completely broke this
00:22:55
linear chain into a branch structure now
00:22:58
this and that are not stereoisomers
00:23:01
these are completely different
00:23:04
molecules in terms of
00:23:06
isomerism they don't fall in a
00:23:08
particular class
00:23:09
they're still eyes on us but these
00:23:12
classes are some specific types that
00:23:15
happen rarely
00:23:16
um and
00:23:18
they don't cover the entire
00:23:20
spectrum of isomers that you can have
00:23:23
so
00:23:24
this is a very special case where you
00:23:26
have only the side groups and their
00:23:29
position
00:23:31
are the
00:23:33
distinctive feature of these isomers and
00:23:36
and what makes them different so
00:23:39
um
00:23:40
another class is
00:23:42
uh isomerism of the kind cis and trans
00:23:45
now
00:23:46
understand this better
00:23:49
each polymer has a main chain so it's
00:23:52
the backbone of the polymer that runs
00:23:55
through and in this case you've got
00:23:57
a chain going there and a chain going
00:24:00
here
00:24:01
now
00:24:02
about that chain you have other side
00:24:05
groups so for instance here you have
00:24:09
a hydrogen side group and a methyl side
00:24:12
group
00:24:13
so here you have these two on opposing
00:24:17
sides here you have them on the same
00:24:19
side
00:24:20
as simple as that the idea is that
00:24:23
these two are again isomers
00:24:25
but one of them has these side groups on
00:24:28
opposing sides
00:24:30
one of them has them both
00:24:32
on one side so
00:24:34
again this may not be a significant
00:24:37
issue
00:24:38
if these were able to rotate
00:24:40
but since the side group here is a large
00:24:43
one it limits the rotation and apologies
00:24:46
this is actually supposed to be a single
00:24:48
bond it's not a double bond there's been
00:24:50
a mistake there and it is not because of
00:24:53
the double bond that the rotation is
00:24:55
limited it is because the side group is
00:24:58
actually rather large and so it's
00:25:01
imposing the constraint on how much this
00:25:02
can rotate
00:25:04
and it's interesting to know that just
00:25:07
this very subtle change
00:25:10
in the way the configuration of these
00:25:12
ponders are can induce a significant
00:25:15
change in their property so
00:25:17
one
00:25:19
turns out to be natural rubber
00:25:21
so it's a cis isoprene and the other one
00:25:24
is gutta-percha the trans isoprene and
00:25:29
you may not have heard of this but
00:25:31
um
00:25:32
this is what what you have this is
00:25:34
natural rubber which you
00:25:38
extremely deformable in soft
00:25:41
this has got to perch a very rigid and
00:25:44
stiff
00:25:45
material in the old days they used to
00:25:48
use this it was derived from a tree
00:25:52
drive to chemical and so
00:25:55
it was sorry it was manufactured with a
00:25:57
tree drive chemical so it was very
00:25:59
bio-friendly but
00:26:01
at the same time it had some very
00:26:03
interesting products they would use this
00:26:05
for
00:26:06
manufacturing golf balls
00:26:08
extremely hard extremely impact
00:26:10
resistant and um at the same time rather
00:26:13
rigid and
00:26:15
hard
00:26:16
so you can see very distinct performance
00:26:19
coming out of just the configuration
00:26:21
being different
00:26:22
another use case for this is now for
00:26:25
root canals in indented stream you can
00:26:27
look at those um in the literature
00:26:30
so
00:26:32
now we're going to talk about the
00:26:34
molecular structures in terms of their
00:26:37
chain configuration
00:26:39
so
00:26:40
molecules can be configured in many ways
00:26:43
so specifically polymers
00:26:46
they could be linear like what i showed
00:26:48
you
00:26:50
a few slides ago when i was showing the
00:26:52
octane
00:26:53
uh if you remember this is a linear
00:26:56
change so all the carbons on the same
00:26:59
chain and it forms a very long chain
00:27:02
similarly you can have branched chains
00:27:04
like the other ones that you have here
00:27:07
instead of having them all on the single
00:27:09
line you can have branches splitting out
00:27:11
coming out on site now
00:27:14
these are two
00:27:16
of these classes so you can have linear
00:27:19
polymers you can have branch polymers
00:27:21
but then these polymers can also have
00:27:24
crosstalk with each other so what that
00:27:26
means is that they can bond together
00:27:30
in order to form a cross-linked network
00:27:32
so you can have these long chains of
00:27:34
polymers that are then connected
00:27:36
together to form a rather dense network
00:27:39
or mesh
00:27:41
which is a cross-linked
00:27:43
network now if it's if it's a lot more
00:27:45
organized than that it becomes a network
00:27:48
when it's got a repeated unit
00:27:51
so
00:27:52
they can just be randomly cross-linked
00:27:54
as they meet each other or they could be
00:27:57
network
00:27:58
now don't
00:28:00
mistake this with non-covalent bonding
00:28:03
because these polymer strands could be
00:28:06
um connected together with some very
00:28:09
loose chemical bonds called non-covalent
00:28:12
bonds
00:28:13
these are actually covalent bonds
00:28:15
meaning that these are connected
00:28:17
together with an actual chemical bond
00:28:20
that keeps them together as in
00:28:22
an entire mesh altogether
00:28:25
and you can imagine that the strength of
00:28:27
these polymers can increase
00:28:29
by increasing the cross-linking density
00:28:32
and network
00:28:34
basically
00:28:36
formation
00:28:38
another interesting
00:28:41
topic the length of these polymers or
00:28:44
the end to end distance
00:28:46
now
00:28:48
if you remember
00:28:50
from the conformations part which i just
00:28:53
had a few slides ago
00:28:56
these molecules here
00:28:58
these large polymers can
00:29:00
rotate in space and although these cases
00:29:03
and so it can go from a linear long
00:29:05
linear molecule
00:29:07
a molecule that bends and twists in
00:29:10
space and has these very
00:29:12
weirdly different
00:29:14
tangled conformations
00:29:16
so based on how these molecules are
00:29:19
entangled
00:29:21
you can have a very long
00:29:24
spirally tangly web that starts from one
00:29:28
point and
00:29:29
the other
00:29:30
so it is not a trivial
00:29:32
process to calculate the length of the
00:29:36
problem as you can see here
00:29:38
there are some simple uh formulae to do
00:29:41
that
00:29:42
but since it's it's a rather broad topic
00:29:45
and it requires simulation a lot of
00:29:47
these cases there's no need to go into
00:29:50
depth there but you can imagine that
00:29:52
there are ways to compute that if you
00:29:54
know the length of each bond and if you
00:29:57
know the angles you can actually then
00:30:00
predict how long this can be or at least
00:30:02
get a distribution of lengths for the
00:30:05
end to end distance
00:30:07
but i'm going to leave it at that here
00:30:11
so
00:30:12
lastly we're going to talk about
00:30:14
copolymers so
00:30:16
copolymers are the case where you have
00:30:20
two or more monomers polymerizing
00:30:23
together
00:30:24
so what if instead of the same monomer
00:30:27
you have different types of monomers
00:30:30
connecting together to form a polymer so
00:30:34
what do i mean so what i mean here is
00:30:37
that you can have
00:30:39
two different types of monomers a and b
00:30:42
to alternate within the same backbone
00:30:46
so here you've got a very nice pattern
00:30:50
you've got one a
00:30:52
followed by one b and then again one a
00:30:54
one b so you've got an alternating
00:30:56
pattern it can also be randomly engaged
00:30:59
within the same chain so you've got a
00:31:01
number of
00:31:02
molecule a followed by number of
00:31:04
molecule b and so on so forth so forth
00:31:08
and you could also have a block
00:31:10
configuration where you have
00:31:12
um
00:31:13
a number of a's together
00:31:16
followed by a number of b's and so there
00:31:19
are lots of ways they can do this but
00:31:20
this one's a special one this is a graph
00:31:23
structure and what it means is that you
00:31:25
first probably form this backbone of a
00:31:29
and then you have these branched
00:31:32
b
00:31:33
chains connecting to the a
00:31:36
and
00:31:36
this actually induces a lot of
00:31:38
interesting
00:31:40
properties because you've got a
00:31:41
secondary
00:31:43
copolymer that's actually been added to
00:31:46
your your main
00:31:48
polymer configuration probably
00:31:50
in a sequential process once you've
00:31:52
already synthesized this now you're
00:31:54
adding this
00:31:55
and it produces some rather interesting
00:31:58
properties hopefully we can get to some
00:31:59
of that in the future
00:32:03
so
00:32:06
talking about polymers their
00:32:07
configuration geometry
00:32:10
we've already discussed copolymers and
00:32:13
the molecular structure
00:32:15
distance uh the next topic is polymer
00:32:18
crystallinity
00:32:19
and
00:32:20
before we get there i'm going to give
00:32:22
you a break this
00:32:24
another long lecture so
00:32:26
we will leave it here and we will
00:32:28
continue talking about polymer
00:32:30
crystallinity
00:32:31
in the next lecture
00:32:34
um
00:32:35
talk to you soon bye
