3 SM Glassy Polymers Copolymer SelfAssembly and Polymers

3 SM Glassy Polymers, Copolymer Self-Assembly and Polymers in Solutions 6 May, 2010 Lecture 9 See Jones’ Soft Condensed Matter, Chapt. 5 & 9

Polymer Conformation in Glass In a “freely-jointed” chain, each repeat unit can assume any orientation in space. Shown to be valid for polymer glasses and melts. Describe as a “random walk” with N repeat units (i. e. steps), each with a size of a: N 3 2 1 a N i=1 The average R for an ensemble of polymers is 0. But what is the mean-squared end-to-end distance, ?

Random Walk Statistics N i=1 N q 34 j=1 q 12 q 23 By definition: Those terms in which i=j can be simplified as: NN i j The angle can assume any value between 0 and 2 p and is uncorrelated. Therefore: Finally, Compare to random walk statistics for colloids!

Defining the Size of Polymer Molecules We see that and (Root-mean squared end-to-end distance) Often, we want to consider the size of isolated polymer molecules. In a simple approach, “freely-jointed molecules” can be described as spheres with a characteristic size of Typically, “a” has a value of 0. 6 nm or so. Hence, a very large molecule with 104 repeat units will have a r. m. s. end-to -end distance of 60 nm. On the other hand, the contour length of the same molecule will be much greater: a. N = 6 x 103 nm or 6 mm!

Scaling Relations of Polymer Size Observe that the rms end-to-end distance is proportional to the square root of N (for a polymer glass). Hence, if N becomes 9 times as big, the “size” of the molecule is only three times as big. If the molecule is straightened out, then its length will be proportional to N.

Concept of Space Filling Molecules are in a random coil in a polymer glass, but that does not mean that it contains a lot of “open space”. Instead, there is extensive overlap between molecules. Thus, instead of open space within a molecule, there are other molecules, which ensure “space filling”.

Distribution of End-to-End Distances In an ensemble of polymers, the molecules each have a different end-to-end distance, R. In the limit of large N, there is a Gaussian distribution of end-to-end distances, described by a probability function (number/volume): Larger coils are less probable, and the most likely place for a chain end is at the starting point of the random coil. Just as when we described the structure of glasses, we can construct a radial distribution function, g(r), by multiplying P(R) by the surface area of a sphere with radius, R:

P(R) g(R) From U. Gedde, Polymer Physics

Entropic Effects Recall the Boltzmann equation for calculating the entropy, S, of a system by considering the number of microstates, , for a given macro-state: S = k ln In the case of arranging a polymer’s repeat units in a coil shape, we see that = P(R), so that: If a molecule is stretched, and its R increases, S(R) will decrease (become more negative). Intuitively, this makes sense, as an uncoiled molecule will have more order (be less disordered).

Concept of an “Entropic Spring” Fewer configurations R R Decreasing entropy Helmholtz free energy: F = U - TS Internal energy, U, does not change significantly with stretching. Restoring force, f

Difference between a Spring and a Polymer Coil f x Spring f In experiments, f for single molecules can be measured using an AFM tip! Polymer Entropy (S) change is negligible, but DU is large, providing the restoring force, f. S change is large; it provides the restoring force, f.

Molecules that are Not-Freely Jointed In reality, most molecules are not “freely-jointed” (not really like a pearl necklace), but their conformation can still be described using random walk statistics. Why? (1) Covalent bonds have preferred bond angles. (2) Bond rotation is often hindered. In such cases, g monomer repeat units can be treated as a “statistical step length”, s (in place of the length, a). A polymer with N monomer repeat units, will have N/g statistical step units. The mean-squared end-to-end distance then becomes:

Example of Copolymer Morphologies Immiscible polymers can be “tied together” within the same diblock copolymer molecules. Phase separation cannot occur on large length scales. 2 mm x 2 mm Poly(styrene) and poly(methyl methacrylate) diblock copolymer Poly(ethylene) diblock copolymers

Self-Assembly of Di-Block Copolymers Diblock copolymers are very effective “building blocks” of materials at the nanometer length scale. They can form “lamellae” in thin films, in which the spacing is a function of the sizes of the two blocks. At equilibrium, the block with the lowest surface energy, g, segregates at the surface! The system will become “frustrated” when one block prefers the air interface because of its lower g, but the alternation of the blocks requires the other block to be at that interface. Ordering can then be disrupted.

Thin Film Lamellae: Competing Effects The addition of each layer creates an interface with an energy, g. Increasing the lamellar thickness reduces the free energy per unit volume and is therefore favoured by g. d Increasing the lamellar thickness, on the other hand, imposes a free energy cost, because it perturbs the random coil conformation. There is thermodynamic competition between polymer chain stretching and coiling to determine the lamellar thickness, d. The value of d is determined by the minimisation of the free energy. Poly(styrene) and poly(methyl methacrylate) copolymer

Interfacial Area/Volume e d=e/3 Area of each interface: A = e 2 Lamella thickness: d e In general, d = e divided by an integer value. Interfacial area/Volume:

Determination of Lamellar Spacing • Free energy increase caused by chain stretching (per molecule): Ratio of (lamellar spacing)2 to (random coil size)2 • The interfacial area per unit volume of polymer is 1/d, and hence the interfacial energy per unit volume is g/d. The volume of a molecule is approximated as Na 3, and so there are 1/(Na 3) molecules per unit volume. • Free energy increase (per polymer molecule) caused by the presence of interfaces: Total free energy change: Ftot = Fstr + Fint

Free Energy Minimisation F Fstr Fint Ftot Two different dependencies on d! Finding the minimum, where slope is 0: d Chains are NOT fully stretched but nor are they randomly coiled! The thickness, d, of lamellae created by diblock copolymers is proportional to N 2/3. Thus, the molecules are not fully-stretched (d ~ N 1) but nor are they randomly coiled (d ~ N 1/2).

Experimental Study of Polymer Lamellae Small-angle X-ray Scattering (SAXS) Transmission Electron Microscopy Poly(styrene)-bpoly(isoprene) (°) T. Hashimoto et al. , Macromolecules (1980) 13, p. 1237.

Support of Scaling Argument 2/3 T. Hashimoto et al. , Macromolecules (1980) 13, p. 1237.

Micellar Structure of Diblock Copolymers When diblock copolymers are asymmetric, lamellar structures are not favoured – as too much interface would form! Instead the shorter block segregates into small spherical phases known as “micelles”. Interfacial “energy cost”: g(4 pr 2) Density within phases is maintained close to the bulk value. Reduced stretching energy when the shorter block is in the micelle.

Copolymer Micelles AFM image 5 mm x 5 mm Diblock copolymer of poly(styrene) and poly(vinyl pyrrolidone): poly(PS-b-PVP)

Diblock Copolymer Morphologies Gyroid TRI-block Lamellar Pierced Lamellar Cylindrical Gyroid Spherical micelle Diamond “Bow-Tie”

Copolymer Phase Diagram N ~10 f From I. W. Hamley, Intro. to Soft Matter, p. 120.

Applications of Self-Assembly Thin layer of poly(methyl methacrylate)/ poly(styrene) diblock copolymer. Image from IBM (taken from BBC website) Nanolithography to make electronic structures Creation of “photonic band Images from website of Prof. Ned Thomas, MIT gap” materials

Nanolithography Used to make nanosized “flash memories” From Scientific American, March 2004, p. 44

The Self-Avoiding Walk In describing the polymer coil as a random walk, it was tacitly assumed that the chain could “cross itself”. The conformation of polymer molecules in a polymer glass and in a melted polymer can be adequately described by random walk statistics. But, when polymers are dissolved in solvents (e. g. water or acetone), they are often expanded to sizes greater than a random coil. Such expanded conformations are described by a “selfavoiding walk” in which <R 2>1/2 is given by a. Nn (instead of a. N 1/2 as for a coil described by a random walk). What is the value of n?

Excluded Volume Paul Flory developed an argument in which a polymer in a solvent is described as N repeat units confined to a volume of R 3. Each repeat unit prevents other units from occupying the same volume. The entropy associated with the chain conformation (“coil disorder”) is decreased by the presence of the other units. There is an excluded volume! From the Boltzmann equation, we know that entropy, S, can be calculated from the number of microstates, , for a macrostate: S = k ln . In an ideal polymer coil with no excluded volume, is inversely related to the number density of units, r : where c is a constant

Entropy with Excluded Volume Hence, the entropy for each repeat unit in an ideal polymer coil is Unit vol. = b Nth unit In the non-ideal case, however, each unit is excluded from the volume occupied by the other N units, each with a volume, b: But if x is small, then ln(1 -x) -x, so: R

Excluded Volume Contribution to F For each unit, the entropy decrease from the excluded volume will lead to an increase in the free energy, as F = U - TS: Of course, a polymer molecule consists of N repeat units, and so the increase in F for a molecule, as a result of the excluded volume, is Larger R values reduce the free energy. Hence, expansion is favoured by excluded volume effects.

Elastic Contributions to F Earlier in the lecture, however, we saw that the coiling of polymer molecules increased the entropy. This additional entropy contributes an elastic contribution to F: Reducing the R by coiling will decrease the free energy. Coiling up of the molecules is therefore favoured by elastic contributions.

Total Free Energy of an Expanded Coil The total free energy change is obtained from the sum of the two contributions: Fexc + Fel Ftot Fel Fexc R At equilibrium, the polymer coil will adopt an R that minimises Ftot. At the minimum, d. Ftot/d. R = 0:

Characterising the Self-Avoiding Walk Re-arranging: So, The volume of a repeat unit, b, can be approximated as a 3. This result agrees with a more exact value of n obtained via a computational method: 0. 588 Measurements of polymer coil sizes in solvent also support theoretical (scaling) result. But when are excluded volume effects important?

Visualisation of the Self-Avoiding Walk 2 -D Random walks 2 -D Self-avoiding walks

Polymer/Solvent Interaction Energy So far, we have neglected the interaction energies between the components of a polymer solution (polymer + solvent). Units in a polymer molecule have an interaction energy with other nearby (non-bonded) units: wpp wss wps There is similarly an interaction energy between the solvent molecules (wss). Finally, when the polymer is dissolved in the solvent, a new interaction energy between the polymer units and solvent (wps) is introduced.

Polymer/Solvent -Parameter When a polymer is dissolved in solvent, new polymer-solvent (ps) contacts are made, while contacts between like molecules (pp + ss) are lost. Following arguments similar to our approach for liquid miscibility, we can derive a -parameter for polymer units in solvent: where z is the number of neighbour contacts per unit or solvent molecule. We note that N/R 3 represents the concentration of the repeat units in the “occupied volume”, and the volume of the polymer molecule is Nb. When a polymer is added to a solvent, the change in potential energy, (from the change in w) will cause a change in internal energy, DU: Observe that smaller coils reduce the number of P-S contacts because more P -P contacts are created. For a +ve c, DUint is more negative and F is reduced.

Significance of the -Parameter We recall that excluded volume effects favour coil swelling: Opposing the swelling will be the polymer/solvent interactions, as described by DUint. (But also - elastic effects, in which Fel ~ R 2, are also still active!) As the form of the expressions for Fexc and DUint are the same, they can be combined into a single equation: The value of c then tells us whether the excluded volume effects are significant or whether they are counter-acted by polymer/solvent interactions.

Types of Solvent • When c = 1/2, the two effects cancel: Fexc + DUint = 0. The coil size is determined by elastic (entropic) effects only, so it adopts a random-coil conformation. The solvent is called a “theta-solvent”. • When c < 1/2, the term is positive, and the excluded volume/energetic effects contribute to determining the coil size: Fexc + DUint > 0. as shown previously (considering the balance with the elastic energy). The molecule is said to be swollen in a “good solvent”.

Types of Solvent When c > 1/2, the term goes negative, and the polymer/solvent interactions dominate in determining the coil size. Fexc + DUint < 0. Energy is reduced by coiling up the molecule (i. e. by reducing its R). Elastic (entropic) contributions likewise favour coiling. Both terms lower F (which is favourable) as R decreases. The molecule forms a globule in a “bad solvent”.

Determining Structure: Scattering Experiments l d = characteristic spacing Scattered intensity is measured as a function of the wave vector, q:

Determination of Polymer Conformation Scattering Intensity, I q -1/n or I -1 q 1/n Good solvent: I q 1/(3/5) Theta solvent: I q 1/(1/2)

Applications of Polymer Coiling Nano-valves Bad solvent: “Valve open” Good solvent: “Valve closed” Switching of colloidal stability Bad solvent: Unstabilised Good solvent: Sterically stabilised

A Nano-Motor? • The transition from an expanded coil to a globule can be initiated by changing . A possible “nano-motor”! c > 1/2 c < 1/2 Changes in temperature or p. H can be used to make the polymer coil expand contract.

Polymer Particles Adsorbed on a Positively-Charged Surface 1 mm 100 nm Particles can contain small molecules such as a drug or a flavouring agent. Thus, they are a “nano-capsule”.

Comparison of Particle Response in Solution and at an Interface Ellipsometry of adsorbed particles Bad solvent: particle is closed Good solvent: particle is open Light scattering from solution

Radius of Gyration of a Polymer Coil The radius of gyration is the root-mean square distance of an objects' parts from its center of gravity. R R For a hard, solid sphere of radius, R, the radius of gyration, Rg, is: A polymer coil is less dense than a hard, solid sphere. Thus, its Rg is significantly less than the rms-R:

Interfacial Width, w, between Immiscible Polymers A B loop w • Consider the interface between two immiscible polymers (A and B), such as in a phase-separated blend or in a diblock copolymer. • The molecules at the interface want to maximise their entropy by maintaining their random coil shape. • Part of the chain - a “loop” – from A will extend into B over a distance comparable to the interfacial width, w. Our statistical analysis predicts the size of the loop is ~ a(Nloop)1/2

Simple Scaling Argument for Polymer Interfacial Width, w But every unit of the “A” molecule that enters the “B” phase has an unfavourable interaction energy. The total interaction energy is: At equilibrium, this unfavourable interaction energy will be comparable to thermal energy: In which case: Substituting in for Nloop: