J.S. HIGGINS1, S.A.BUTLER1, D.G.BUCKNALL2
1Department of Chemical Engineering, Imperial College, Prince Consort Road, London, SW7 2BY, U.K.
2Department of Materials, Oxford University, Parks Road, Oxford, OXON, OX1 3PH, U.K.
Neutron reflectivity (NR) has been widely exploited to look at polymer thin films and in many ways is an ideal technique for studying polymer interfaces and surfaces, providing high resolution concentration depth profiles across the film thickness. Most NR studies to date have concentrated on thin films of amorphous polymers which possess Tg values well above room temperature. These polymers are ideally suited to NR measurements, firstly because they form homogeneously flat films, but secondly, heat-quench cycles can be used to study time dependent processes. This has been used to great effect in NR studies of the initial stages of polymer-polymer interdiffusion or the kinetics of surface segregated layers for instance.
One of the biggest drawbacks to this approach is that in polymer systems where one or more of the components has a Tg close to or less than room temperature, the polymers can still move during the measurement time of an NR profile, which typically takes 1-2 hours for a full profile. Therefore, in order to study such systems we have developed an approach to NR reflectivity measurements which allows us to investigate diffusion processes in-situ. Our new approach allows us to take NR profiles in only 15 seconds. This paper describes the method of real time NR measurements in detail and illustrates the capabilities of the technique with highlights from some of our recent work on the early stages of polymer-plasticiser interdiffusion.
SCATTERING STUDIES OF UNIVERSALITY AND HIERARCHICAL STRUCTURE IN POLYMER MIXTURES UNDERGOING SPINODAL DECOMPOSITION
T. HASHIMOTO a, b), H. JINNAI a, c), T. KOGA a, b)
aHashimoto Polymer Phasing Project, ERATO, JST, Japan,
bDepartment of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan, and
cDepartment of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan
We overview time evolution of phase-separating structure in binary mixtures as observed for a wide range of wave number q for Fourier modes by a combined technique of time-resolved small-angle neutron scattering (SANS) and light scattering as well as laser-scanning confocal microscopy. The binary mixtures to be discussed here are those in which two components are symmetric, having nearly identical molecular volumes and dynamical properties, and nearly equal volume fraction. Such mixtures undergo phase separation via spinodal decomposition (SD). By taking an advantage of using polymers as component molecules we could elucidate time evolution of hierarchical structures comprised of global structure, interface structure, and local composition fluctuations within separated two phases in the order of increasing q. The global structure is highlighted to have a periodic co-continuous two-phase structures as a structure universal to symmetric polymer mixtures and simple-liquid mixtures. It is characterized by a sponge-like structure with a hyperbolic interface having negative Gaussian curvature K and zero area-averaged mean curvature <H>. The sponge-like structure is found to be theoretically predicted well by using computer simulations based on a time-dependent Ginzburg-Landau model.
An analysis of the interface structure by SANS unveiled ”interphase scattering” at a large q range and at a late stage SD that originates from composition fluctuations within ”interphase” (interfacial region with a finite interfacial thickness). This interphase scattering found for the first time gives the scattering excess to the Ornstein-Zernike scattering arising from the local composition fluctuations within the two phases and is remarkably unveiled as the scattering from the interface which obeys the Porod low becomes negligibly small with time in the q-range of the observation.
We shall briefly discuss some unique features brought by a dynamical asymmetry in the component molecules which involves a stress-diffusion coupling. It is striking to note that in such binary systems viscoelasticity plays an important role on phase-separation process.
CRYSTALLISATION IN BLOCK COPOLYMERS
A.J. RYANa, J.P.A.FAIRCLOUGHa, S.-M. MAIa,
S.C.TURNERa, J. XUa,
C. BOOTHb, I.W.HAMLEYc, R.A. REGISTERd,
L. LOOd, W. BRASe,
A.J. GLEESONf, N.J. TERRILLf
aDepartment of Chemistry, University of Sheffield, Sheffield, S3 7HF, U.K.
bDepartment of Chemistry, University of Manchester, Manchester, M13 9PL, U.K.
cSchool of Chemistry, University of Leeds, Leeds, LS2 9GT, U.K.
dDepartment of Chemical Engineering, Princeton University, NJ 08544, USA
eDUBBLE CRG, ESRF, F38043, Grenoble Cedex, France
fCLRC Daresbury Laboratory, Warrington, WA4 4AD U.K.
Block copolymers are macromolecules comprised of two or more chemically different chains joined together by covalent bonds. A variety of molecular architectures are possible by anionic polymerisation, e.g. AB diblock, ABA triblock, cyclic and starblock copolymers. The material (bulk) properties of many block copolymers are dominated by their tendency to spontaneously separate into microphases when the temperature is lowered. The microphases exhibited by block copolymers have been studied by many different groups, and theorists and experimentalists have combined to make the phase behaviour of block copolymers in the liquid state well understood. The crystallisation of block copolymers presents a unique opportunity to study a number of aspects of polymer structure and dynamics and is far less well understood. This lecture will present a review of the phenomena encountered in crystallisation of block copolymers.
In general the energy associated with crystallisation is far greater than that associated with phase separation and there is a competition between kinetics and thermodynamics that defines the structure formed. The relationship between the order-disorder transition, glass transition and crystallisation temperatures of the block copolymers, combined with mass transport, sets the semi-crystalline structure. The behaviour maybe organised into two broad classes, confined- and break-out-crystallisation, with subtle features in structure and dynamics within each class. In confined-crystallisation the process occurs within the block copolymer domains where either the amorphous block is glassy or the microphase separation process is strong and morphological rearrangement is not possible in the time-scale of the crystallisation process. In break-out-crystallisation the energetics dominate and massive reorganisation is associated with the formation of semicrystalline lamellae with the potential for equilibrium chain folded structures. There are subtleties associated with chain conformation that cause epitaxial relations in some cases and anomalous liquid structures are seen when the order-disorder transition and crystallisation temperatures are in close proximity.
Structure and Dynamics of Silica-filled Polymers by SANS and coherent SAXS
Erik GEISSLER, Anne-Marie HECHT, Cyrille ROCHAS, Ferenc HORKAY, Françoise BLEY and Frédéric LIVET
Laboratoire de Spectrométrie Physique UMR CNRS 5588, Université J.Fourier de Grenoble, B.P.87, 38402 St Martin d’Hères cedex, France
Laboratory of Integrative and Medical Biophysics, National Institutes of Health, 13 South Drive, Bethesda MD 20892, USA
Laboratoire de Thermodynamique et Physico-chimie métallurgiques, CNRS UMR 4777, INPG, 38402 St Martin d'Hères cedex, FRANCE.
Random cross-linking in elastomers gives birth to local variations in the cross-link density. When the network is swollen in a low molecular weight solvent, competition between the osmotic pressure and the local elastic constraints transforms these variations into differences in polymer concentration, the range and amplitude of which can be measured by small angle X-ray or neutron scattering (SAXS or SANS). In polymers containing a filler, the distribution of polymer, as well as of the elastic constraints, is similarly modified. SANS measurements, by varying the proportion of deuterated solvent molecules in the network, allow the scattering function of the polymer to be distinguished from that of the filler. Such measurements can be used to yield not only the internal surface area of the filler particles but also the fraction of that surface in contact with the polymer.
The recently developed technique of quasi-elastic SAXS detects slow dynamic processes at wave vectors larger than those accessible with visible light lasers. This technique is used to investigate the dynamics of filler particles in uncross-linked polymer melts. It is shown directly that the structural reorganization process of the filler following an external mechanical perturbation is diffusion controlled.
vNEW RESULTS FOR SPHERE-FORMING BLOCK COPOLYMERS
TIMOTHY P. LODGE, ELENA E. DORMIDONTOVA, XIAOHUI WANG, JOONA BANG, AND KENNETH J. HANLEY
Block copolymers are well-known to self-assemble into approximately spherical aggregates, or micelles, under at least two conditions: in selective solvents at low and moderate concentrations, and in the bulk when the copolymer composition is strongly asymmetric. In both circumstances interesting questions persist, particularly in the vicinity of the order-disorder transition (ODT). For example, for several styrene-isoprene diblocks in dialkylphthalate solutions we have found regions of concentration and temperature where a suspension of micelles orders on a lattice upon heating, before disordering again at higher temperatures. In other words there is both a lower (LODT) and an upper (UODT) transition. Furthermore, the intervening ordered state symmetry is either fcc or bcc, and in some cases a thermotropic, reversible fcc/bcc transition is found. A complete explanation for this behavior is so far lacking, but we note that it is not consistent with a previously proposed criterion for fcc/bcc selection in the literature.
In the case of asymmetric molten copolymers the ordered state symmetry is apparently always bcc. Self-consistent mean-field (SCMF) theory anticipates a narrow window of fcc packing before the ODT, but this has never been seen experimentally. Rather, the ODT corresponds to a transition from a bcc lattice to a disordered array of micelles. We have extended the strong-segregation scaling theory of Semenov, and found that the ODT occurs from bcc into a ”disordered micelle state” at somewhat higher c N (lower T) than SCMF theory anticipated. We demonstrate the thermodynamic preference of this ”disordered micelle state” over the fcc phase in this region. This is consistent with experimental results from several groups. The range of temperature over which the fraction of micelles in the disordered micelle state remains significant (to be distinguished from a totally disordered unimer melt), is limited by a critical micelle temperature, CMT. This is not a phase transition from a thermodynamic point of view, and whether it can be identified experimentally remains unclear. Measurements on ethylenepropylene-dimethylsiloxane diblocks over a wide range of temperature, by SAXS, SANS, and rheology, indicate that the micelles may persist as much as 100 degrees above the ODT. Furthermore, there is not a sharp transition to a fully disordered state; rather, the behavior resembles a broad mean-field/non-mean-field crossover, as observed for symmetric diblocks.
STRUCTURE AND DYNAMICS IN AQUEOUS POLYMER SOLUTIONS STUDIED BY SAXS AND DLS
O. GLATTER, H.LINDNER, A. BERGMANN, J. INNERLOHINGER, G. SCHERF
Institute of Chemistry, University of Graz, Heinrichstrasse 28, A-8010 Graz, Austria
In this contribution we present our studies on aqueous solutions of triblock copolymers (Synperonic® P94). This PEO-PPO-PEO polymer shows a rich and interesting phase behavior in aqueous solution including unimeric and micellar phases with spherical and rod-like structures as a function of temperature. With increasing concentration this system transforms into a clear stiff gel (cubic phase) at ambient temperatures or into a clear soft gel (hexagonal phase) at higher temperatures. The structure in these solutions has been investigated by SAXS and SANS (small-angle X-ray and neutron scattering) while the dynamics have been investigated by DLS (dynamic light scattering).
In terms of structures it was of special interest to learn about micellar interactions and effective volume fractions occupied by the micelles. It was found that these micelles can be described by micelles of constant size, interacting like hard spheres with a volume fraction of twice the weight fraction. The particle form factor was determined without any model assumptions using the recently developed Generalized Indirect Fourier Transformation (GIFT) method, the structure factor is modeled with a polydisperse hard sphere model using Perkus Yevick closure relation. Both, the form factor and the structure factor, can so be directly determined from the measured scattering function1.
The diffusion behavior (DLS) can also be described by hard spheres, there is no sign of increased friction by overlapping PEO chains like it is observed with inverse surfactant micelles. At higher temperatures the micelles grow into long rods. Rotational diffusion (Depolarized DLS) can be used to estimate the mean length of these micelles.
In the cubic phase we are dealing with a non-ergodic system. The clear gel allows measuring diffusional behavior in this system. The transition into the gel can be established by increasing the concentration at a given temperature or, more simply, by increasing the temperature at a given concentration, i.e. going from a unimeric state into a micellar state. This makes such polymer solutions an interesting test case for studies on gelation and glass transitions.
1 Bergmann, A., Fritz, G., and Glatter, O. J. Appl. Cryst. (2000) 33, 1212 -1216. ”Solving the Generalized Indirect Fourier Transformation (GIFT) by Boltzmann Simplex Simulated Annealing (BSSA)”.
Polymer Brushes and Mushrooms in Polymeric Matrices*
SPIROS H. ANASTASIADIS†
Foundation for Research and Technology - Hellas, Institute of Electronic Structure and Laser, P. O. Box 1527, 711 10 Heraklion Crete, Greece and University of Crete, Physics Department, P. O. Box 2208, 710 03 Heraklion Crete, Greece
The interfacial segregation of diblock copolymers to the substrate/polymer interface from their mixtures with the respective homopolymers in thin films is investigated by neutron reflectivity; the adsorbed chain configuration is probed as a function of the ratio of block lengths. The segment density profiles of PV2P-PS diblocks adsorbed at the PS/substrate interface as a function of the size of the anchoring block are evaluated. Besides, the composition profiles and conformational characteristics of all chain species present in the interfacial region are investigated using a lattice-based self-consistent field model inspired by the work of Scheutjens and Fleer as extended to incorporate chain conformational stiffness. Inputs to the model are the molecular and macromolecular characteristics of the various polymer species, interaction parameters extracted from experimental binary interfacial widths as well as the experimental data on total surface excess. Both experiment and theory reveal evidence for a broad transition from a ”mushroom” to a ”wet brush” configuration of the dangling chains by changing the ratio of the block lengths. Applications of these effects in the development of ”smart” surfaces, i.e. of surfaces that would respond to their environment, are presented. The methodology is demonstrated for surfaces that can alter their wetting characteristics when exposed to water vapor. For this, we take advantage of the surface partitioning of block copolymers at the polymer/air interface and utilize a hydrophilic group at the end of the surface-active block.
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† In collaboration with H. Retsos, D. Anastasopoulos, C. Toprakcioglu, A. Terzis, D. N. Theodorou, G. Smith, A. Menelle, Y. Gallot, G. Hadziioannou, S. Pispas, N. Hadjichristidis, and S. Neophytides
