# Polymer Physics - Radius of Gyration

• I
• ChrisJ
In summary, the conversation discusses the calculation of the radius of gyration for a polymer in equilibrium. The equation for the radius of gyration involves averaging over the chain conformations of the polymer, which can be done over time or over an ensemble of polymer molecules. There is confusion about the notation and what is being averaged, but it is clarified that (ri - rG)2 means the square of the length of the vector ri - rG. The conversation also mentions the use of different notation and the need to check multiple sources for clarification.
ChrisJ
I was not sure if this was the best place for this, it could fit here, in the Chemisty section or the Programming section. So feel free to move if needed.

Essentially I have been modeling polymers in python and using a Monte Carlo, Metropolis type algorithm, to minimise its energy into equilibrium from its intial state governed by a 3D random walk (not on a lattice).

I don't know why I am having so much trouble with this, probably because of different sources calling it different things, or using same notation but using it to refer to different bits. But anyway, all I am trying to do is to calculate the Radius of Gyration for my polymer once it has reached equilibrium.

I have tried to program these two equivalent expressions (below) for the radius of gyration, given in the same textbook, but a couple things confuse me which they don't explain.

##R_g^2 = \frac{1}{N+1} \sum_{i=0}^N \left < ( \textbf{r}_i - \textbf{r}_G )^2 \right > ##
where ## \textbf{r}_G ##, the centre of mass, is given by ## \textbf{r}_G = \frac{1}{N+1} \sum_{i=1}^N \textbf{r}_i ##

And also
##R_g^2 = \frac{1}{2(N+1)^2} \sum_{i,j=0}^N \left < ( \textbf{r}_i - \textbf{r}_j )^2 \right > ##

where the angle brackets represent averaging, but I am confused with what is being averaged? In the notation is is clear that ##R_g^2## is a scalar, but following the equation I end up with a vector still, so it must come out of the averaging,

but if one finds the difference between two vectors, you end up with a vector, if one squares a vector you end up with a vector, and then it asks to take the average... Is the radius of gyration only applicable to many instances of a polymer configuration. Like for example if one did 100 different configurations, then the radius of gyration is applicable to that set of 100 configurations, like with the rms distance in random walks?

I was told by my tutor to find the Radius of gyration of my system in equilibrium and plot vs N, yet it takes like 2-3 hours of the code running to get one polymer configuration from its starting point to equilibrium. So to get a decent set of configurations for N, and then do it x10 to get a plot vs N is going to take like a week of my computer @ 100% running code.

Any guidance on the Radius of Gyration is appreciated.

"Since the chain conformations of a polymer sample are quasi infinite in number and constantly change over time, the "radius of gyration" discussed in polymer physics must usually be understood as a mean over all polymer molecules of the sample and over time. That is, the radius of gyration which is measured as an average over time or ensemble:"

Wiki's equation shows the summation within the angular brackets...?

"where the angular brackets <...> denote the ensemble average."

ChrisJ said:
if one squares a vector you end up with a vector,
No. The dot product of a vector with itself is the square of the length of the vector. The cross product of a vector with itself is zero. (ri - rG)2 must mean |ri - rG|2

mjc123 said:
No. The dot product of a vector with itself is the square of the length of the vector. The cross product of a vector with itself is zero. (ri - rG)2 must mean |ri - rG|2

Its funny. I know that relationship like the back of my hand, yet spent hours last night trying to figure this crap out, so caught up in a mix between formal mathematical notation and programming language notation, and the fact that if one just programs the equation using arrays of length three for vectors, squaring an array is a perfectly valid operation (where the components are simply squared). And my textbook doesn't help when it mixes notation, in the same chapter/section from the one I had in my post (standard ()^2 brackets) and ||^2 for different things. Was very tired last night!

Makes sense.

bahamagreen said:

"Since the chain conformations of a polymer sample are quasi infinite in number and constantly change over time, the "radius of gyration" discussed in polymer physics must usually be understood as a mean over all polymer molecules of the sample and over time. That is, the radius of gyration which is measured as an average over time or ensemble:"

Thanks, the averaging over time bit makes more sense to me in my situation. I've gotten so used to relying on textbooks over the internet that sometimes I don't even think to check places like wikipedia.

Thanks, makes a bit more sense now.

bahamagreen said:
Wiki's equation shows the summation within the angular brackets...?

My textbook showed them both, saying they were equivalent.

## 1. What is the radius of gyration in polymer physics?

The radius of gyration is a measure of the size of a polymer chain. It represents the average distance of all the monomers in the chain from the center of mass of the polymer.

## 2. How is the radius of gyration calculated?

The radius of gyration can be calculated using the following formula: Rg = √(1/N∑i=1N (ri - rcm)2), where N is the number of monomers in the chain, ri is the position vector of the ith monomer, and rcm is the position vector of the center of mass of the polymer.

## 3. What is the significance of the radius of gyration in polymer physics?

The radius of gyration is an important parameter in polymer physics as it provides information about the shape and flexibility of a polymer chain. It is also used to determine the molecular weight and conformation of a polymer.

## 4. How does the radius of gyration change with increasing molecular weight?

The radius of gyration tends to increase with increasing molecular weight. This is because as the number of monomers in the chain increases, the chain becomes longer and more extended, leading to a larger radius of gyration.

## 5. How does environmental factors affect the radius of gyration of a polymer?

The radius of gyration can be affected by various environmental factors such as temperature, solvent quality, and presence of additives. For example, an increase in temperature can lead to an increase in the radius of gyration as the polymer chain becomes more flexible. Similarly, a change in solvent quality can cause the polymer to swell or collapse, resulting in a change in the radius of gyration.

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