Polymer relaxation spectroscopy - dynamic aspects of polymers

600

Polymer relaxation spectroscopy - dynamic aspects of polymers Richard A Pethrick The influence dynamic

of microstructure

properties

and solution

phases

theoreticians

and topography

of polymer

molecules

have challenged

on the

in the bulk, thin film

experimentalists

and

in the past year.

Addresses Department

of Pure and Applied

Chemistry,

295 Cathedral Street, Glasgow e-mall: [email protected]

Gl

Current Opinion 31600-605

& Materials

in Solid

Electronic

Identifier:

c; Current

Chemistry

Abbreviations DETA dielectric

State

Ltd ISSN

Science

1998,

thermal

1359-0286

analysis

perfluorodecyl polyisoprene

PMMA PTMO

poly(methyl methacrylate) poly(tetramethylene oxide) rotational isomeric states model

T,

of Strathclyde,

1359-0286-003-00600

FMA PIP

RISM SIS SMA

University

1 XL, UK;

methacrylate

styrene-isoprene-styrene steryl methacrylate glass transition temperature

Introduction ‘I’he wide availability

of dynamic mechanical thermal analysis (Llhl’TA), dielectric thermal analysis (DE’l’,4) and differential scanning calorimetry (IX(:) has made the study of molecular motion in polymeric materials almost a routine process. Despite almost forty years of intense research, there still remains much, in tams of detail, Lvhich is yet to bc understood about the factors lvhich influence the amplitude and location in the frequency/temper3ture domain of the dynamic process associated with conformational change. It would be an impossible task to review all the literature published in the last year (> 3.500 papers) and therefore I have chosen to discuss a few ‘hot’ topics. The theoretical description of polymer dynamics is either based on the chemical structure and embodied in the ‘rotational isomeric states model’ (RIShl) [ 1,3], or it is developed from a continuum statistical/mean field approach [3-S]. A number of attempts have been made to bridge the gap between the RIShl and the continuum approach, but currently no unified theory exists for the whole of polymer dynamics. hluch of the current research is concerned with the development of an understanding of the relative importance of inter- and intramolecular interactions and longer range coupling effects on the overall dynamics of polymer systems. ‘The local motions of the chain backbone, associated with the glass transition temperature (‘I?,), are sensitive to the chemistry of the polymer and its molar mass, whereas viscoelastic processes are usually L\ simple function of molar mass.

Chemical dominated dynamics

aspects of polymer

Iintil recently, RISRI calculations have been exclusively in the domain of theoreticians, however, the \vide a\.ailability of specialist computer packages has made these calculations accessible to polymer scientists. ‘I’hc calculations allow prediction of the statistical distribution bctwcen conformational states and the rates of interchange between these states. ‘I’he predictions arc fairly wcuratc for situations where van der CVaals simple nonbonding interactions define the energy profile associated \vith the conformational states, but are less wtisfactorv when ionic and hydrogen bonding interactions become important. ‘I’he RIShl approach has been used cffectiwly to predict the size of polymer molecules in dilute solution and the dimensions of individual polymer molecules in the solid state [Cl]. Neutron scattering, using contrast enhanccmcnt by selective deutemtion, has confirmed that polymer coils are often close to their ideal size in the solid aate. ‘l’hc potential of neutron scattering for the study of the static and dynamic properties of polymers has recently been reviewed [7]. Quasielastic neutron scattering and electron spin rcsonancc measurements of the chain dynamics of static polypropylene, have detected a fast process that sets in :rt the \‘ogel-Fulcher temperature, ‘I‘,,, and is connected u ith ‘I’, [8’]. The effects of the microstructure on the local motions of the polymer backbone have been identified through the effect of tic and ~UZNSunits on the local d~namits of bulk polybutadicnes (PBS) [Y]. NhlR mcasurcmcnts at room temperature indicate that the effccti\,c monomeric friction coefficient depends on the microstructure. ‘I’he frequencies of the isomeric jump” arc cornpawble, but the amplitude of the jumps are significantly larger in the ci.c units than in the trt/n.s units. Iinderstanding of the way in which changes in the local force fields influence the dynamic properties of blends is an active area of research. ‘T\Yo-dimensional solid state exchange ?H N1\lK spectroscopy of amorphous poly(ethyl methacrylate) (PEMA) and poly(mcthyl methacrylatc) (PhihIA) below and above their respective glass transition temperatures have been performed [lO’]. In both polymers. the main chain mobility below ‘I‘, is coupled to the p-rclauation process, which involves 180” flips of the carboxyl side groups. At the ‘r,, the coupling of the P-process to the main chain motions occurs to a different extent in both polymers. The ester side group reorients faster in PhlhlA than in PRhlA. Similar conclusions with regards the cxtcnt to which side group and main chain motions ha\,c been obtained from studies of amorphous random copolymers of n-propylmethacrylate and n-pentylmethacrylate [ 111. It is apparent that small changes in local chemical interactions

Polymer relaxation

have profound effects on the longer range motions of the polymer backbone and the accurate prediction of these effects presents a challenge to theoreticians. Calculation of the size of a polymer molecule is an important application of the RISM theory [S]. A wide range of novel architectures have been produced which are likely to produce deviation from random coil statistics [12,13,14”,15,16]. Poly(p-phenylene) is a ladder polymer which has been studied by small angle X-ray and neutron scattering, and the effects of the finite dimensions of the repeating units have been assessed [17’]. This ribbonlike polymer has a wormlike chain with a persistence length of 6.5 nm. However, bending fluctuations will destroy the rod-like structure and lead to a wormlike shape of the chain even when the repeating units are stiff moieties. Even a minute distortion of l-2” per repeating unit, will lead to an overall three-dimensional structure for chains with a degree of polymerisation greater than 20-30. Photon correlation spectroscopy has shown that poly(p-phenylene) with flexible dodecyl side chains aggregates in toluene [18’] (Figure 1). Static and dynamic studies show that trimers are formed in semi-dilute solution. In addition to cooperative diffusion and reorientation of the trimers, two additional relaxation processes are observed and have been attributed to the formation of large anisotropic crystalline clusters of size -570 nm. Aggregation is a problem which requires further theoretical consideration, when polymers with high persistence lengths are being considered. The introduction of interactions either between chains or as a consequence of specific solvent interactions leads to new types of dynamic behaviour. Segmented poly(tetramethylene oxide) (PTMO) zwitterionomers and ionenes undergo phase separation [ 191. All the amorphous copolymers, (M, [PTMO segment] < 3 x 10”) display biphasic character with a soft matrix of pure PTMO, (Ta = -77°C) and hard polar domains (Ta = -5 to 40°C). Semicrystalline copolymers, (M, [PTMO segment] > 3 x 103) show phase separation above their melting points (T, - lO-ZS’C). Biphasic morphologies may be interpreted within the multiple-cluster concept. Polyazomethines or Schiff base polymers exhibit many interesting physical properties [ZO,Zl]. DETA and DMTA studies identify three relaxations associated with, respectively; the T,, rotational motion of the rigid mesogenic core (B) and local motions of the decamethylene flexible spacer (r). Semi-empirical calculations have allowed a prediction of the rotational energy barrier associated with the B process [20,21]. Formation of the copper (II) complex produces behaviour which resembles cross-linking (Figure 2). Changes in the molecular motion in blends of poly (ethyleneoxide)-poly(vinyl-phenol) have been studied by W Tip relaxation [‘22] and the motion of individual

spectroscopy

- dynamic aspects of polymers

Pethrick

601

Figure 1

R1 = CsH13

,R2 = 1,4- C6H4- C10H21 Current ODinion m Solid State & Materials Science

Schematic of the generic polyphenylene structures.

homopolymer chains are found to be coupled. Dielectric studies of miscible blends of poly(vinylethylene) (PVE) and polyisoprene (PIP) indicate that the faster process resembles the PIP dynamics. The slower contribution has a bimodal character and involves both the PIP and PVE units. This is evidence of coupling of the motion of different polymer molecules within a blend. Poly(methylacrylate) (PMA) + poly(4-hydroxylstyrene) (P4HS) and poly(4-hydroxystyrene-co-4-methyloxystyrene) (MP4HS) blends have been studied using dielectric relaxation [23]. Increase of the width of the relaxation curves with decreasing PMA content can be correlated with increased coupling between the motion of the dipoles present. Blends of low molecular weight cis-polyisoprene (&-PIP) and polystyrene (PSt) and the system of &-PIP and vinyl polyisoprene (vinyl-PIP) have been investigated [24”]. These polymers exhibit dielectric relaxation due to the segmental motion and, in addition, &PIP exhibits normal mode relaxation. In cis-PIP/PSt blends the normal mode relaxation is proportional to the PIP content, indicating that the blend is homogeneous over a length scale of the end-to-end distance. In c&PIP/vinyl-PIP systems, a complex loss curve is observed indicating weak segregation and a degree of inhomogeneity and coupling of the motion of individual polymer chains. Backbone reorientation of most polymers is influenced by the free volume and measurement of this parameter using positron annihilation lifetime spectroscopy (PALS) has recently been reviewed [ZS’]. The annihilation characteristics of cis-1,4_poly(butadiene) measured over the temperature range, 1.5-313K, exhibit a pronounced break in the rs-T plot at T, [26’]. Two further breaks exist; the first is small in magnitude and occurs below T,, the second is in the elastic state and is associated with the crystalline melt temperature, T,. These data correlate well with neutron scattering measurements. Blends of PMMA and poly(ethyleneoxide) [27] studied between room temperature and 110°C indicate a positive deviation from additivity of the lifetimes and intensities reflecting the biphasic nature of the blends.

602

Polymers

Figure 2 Formation

of the copper

(II) complex.

Current Op~mon I” SolId State & Matwals Science

Physical ageing of amorphous

polymers

‘I‘he importance of physical ageing on the dynamic properties is becoming widely recognised [28,29]. The dielectric (a) relaxation in three amorphous polymers; poly(bisphenolZ-hvdroxvprop~lether), l”)lv(vin\rlacct3te) and poly(vinyl methyl ether) have been irwestig&d [30] and described by the Kohlrausch \\:illiams Watts (KKW) function. ‘l-he relaxation time increases with ageing time but the shape does not. I>ependence of the relaxation time on the configurational entropy \\-a~ observed. Correlation bet\veen IShI’ia, DE’rA and I’AI,S data obtained at different ageing times have been reported for I’NiLlA [31’]. and indicate the connectivity at 3 molecular level of processes responsible for physical ageing. Studies hal-c also been reported on the enthalp~ of relaxation in polystyrene [28,29].

Continuum

polyisobutylenc near and above the glass transition tenperaturc (X”] are described by t\\o 1 iscoelastic proccsacs. ‘l’he temperature dependence of the local segmental relaxation times are stronger than the sub-Kouse mode relaxation timcs. Neutron spin echo and dielectric musurements on the local dynamics of polyisobtltylc~~e [36] agree with the classical rheological terminal zone predictions. Dynamic light scattcring of semi-dilute solutions of polystyrene in benzene [37”] fit the predictions of Semeno\-‘s theory. ‘I’he slowest component \~as identified as the diffusion of clusters with characteristic size in the range of 100 nm. In addition, the broad mode corresponds to the rcptation process and a further diffusil c mode, with a decay time independent of molecular Lvcight, is observed and assigned to dynamics associated with the inter-entanglement spacing.

theory of polymer chains

Properties such as viscosity arc adequately described by theories based on the molar mass. Excellent reviews of the continuum statistical approach [3] and I mathematically rigorous exposition of the theory [-I] ha\~ been complemented by a \u-y readable book ‘Giant hlolecnles’ 1.321. A supporting CD ROhl illustrates the important prediction of continuum theory and allows visualisation of the highly co-operative reptation and crystallisation processes. ‘l’he Rouse and Zimm descriptions of the local motion of isolated chains in dilute solution [3,-l] ha1.e stood the tat of time, and are still widely used to describe the cooperativc motion in solution and the melt phase [33]. Dielectric studies on dipole inverted ,;ls-pol~isoprene in solution [31] have indicated that the method is capable of detecting the global motion of the PIP chain. ‘l’hese observations fit the prediction that the ratio of the ROLISC : reptation relaxation times (7, : z2) should be equal to four. Photocorrelation spectroscopy studies indicate that the dynamics of the local segmental and sub-Rouse motion of a disperse

Rheological and dielectric bchaviour have bcrn examined for concentrated solutions of styrene-isoprenc-st~rcnc (SIS) triblock copolymer in n-tetradecanc, (C:,,, ) and in ;I low hl,, = 1.4 k, homopolyisoprene 1381. ‘I’he 1 blocks had symmetrically once in\wted dipoles along the block contour and were dielectrically acti\.c. ‘l’hc SIS solutions exhibited rubbery, plastic and viscous bchaviour at 1ow, intermediate and high temperatures. ‘I’he S and I blocks were more or less homogeneously mixed in the \iscow regime. whereas in the rubbery and plastic regimes, the S blocks were segregated to form spherical domains anti the I blocks enabled the dielectric estimation of the loop fraction, which WBS equal to 60% in (:14. ‘l’hese loops ha\ ing osmotically

constrained

conformations.

are

strongly

affect-

rheological properties of the SIS solutions. Aggregation, whether it be through specific interactions or through van der Waals forces, introduces complications to the rheological description of the motions of real polymer systems. ed

by

the

Polymer relaxation spectroscopy - dynamic aspects of polymers Pethrick

603

Atomic force microscopy of poly(styrene)/ poly(bromostyrene) [P(Br, $)I. (a) lo%, (b) 509’0, Cc) 60%, (e) 700/, and (d) 90% w/w. Reproduced with permission from 1391.

Dynamic properties of thin films and polymer surfaces There has been much speculation recently about the properties of thin polymer films. Atomic force microscopy and related techniques have allowed visualisation of the surface and exploration of its dynamic properties. The complexities which can exist in the surface structure of polymer blends are illustrated in the studies of deuterated poly(styrene)-poly(bromostyrene) blends, Figure 3 [39]. The different topographical features observed arise during the spinodal decomposition of the polymer mixtures during film formation. Such rough surface structure is being observed for many polymer blends [38] and related neutron scattering studies have been reported at water/air interfaces [40]. Forced modulation scanning force microscopy has been used to explore the surface dynamic storage modulus, E’, and the surface loss tangent, tan 6, of monodispersed polystyrene (M, > 26.6 k) films [41]. The magnitude of E’ and tan 6 for the thin films were respectively, lower and higher, than those for the bulk state. The surface was at its T, at 293K in spite of the bulk T, being above 293K, implying an excess free volume due to the surface segregation of chain-end groups. Near edge X-ray absorption fine structure, (NEXAFS) spectroscopy studies of polystyrene [42], near a free surface after an imposition of a small deformation indicate depth sensitive relaxation behaviour. The dichroic ratio was determined for both the Auger and total electron yield processes as a function of temperature to obtain the orientation of the polymer in the l-10 nm region from the surface. Complete relaxation of the polymer was not observed below T,, which implies that the bulk imposes constraints on the surface of the homopolymer system. The motion of poly(ethylene) chain ends tethered to a fresh surface of poly(tetrafluoroethylene) were studied [43] in uzcuo in the range 2.8-95K by

electron spin resonance. The high mobility of the tethered PE chain ends is attributed to the large space around the chains and a lack of chain aggregation, because the concentration of chain-ends is low and they are tethered. These studies imply that polymer chains in thin films can exhibit additional free volume and higher mobility than when they are part of a bulk system.

Comb-like

architectures

The dynamics of comb-like polymers with a methacrylic backbone and flexible side chains have been studied with dielectric measurements [44]. The cooperative motions of the main chain segments and the side chains were characterised in an amorphous polymer, that is the homopolymer of oleoyl methacrylate (POMA). When chemically similar homopolymers of steryl (PSMA) and perfluorododecyl methacrylate (PFMA) were studied, it was found that the side chains form crystalline lamellae. Above the melting point, the segmental relaxations of PSMA and PFMA are similar to those of the amorphous POMA. Below the melting point, however, the mobility of the methacrylic main chains is almost entirely suppressed, although the chain backbones are not built into the crystalline lamellae of the side chains. Random copolymers of steryl (SMA) and perfluorodecyl (FMA) methacrylate units have been studied. In SMA-FMA, the SMA and FMA side chains crystallise independently in a bilayer structure, the FMA side chains melt at a higher temperature than the SMA side chains. Between the two melting points, the copolymer SMA-FMA consists of molten SMA lamellae and crystalline FMA lamellae. The amplitude of the methacrylic main chain motion was found to increase abruptly at the melting points. Three hyperbranched polyesters with the same backbone structure but with different terminal groups: hydroxyl, benzoate or acetate, have been studied by DETA, DSC and DMTA [45]. The benzoate and

604

Polymers

acetate terminated polymers exhibited only one subglass process (p) associated with the ester groups, and distinct from the ‘l’, process. The hydroxy terminated sample exhibited a low temperature sub ‘l‘, process ($ due to motions of the hydroxyl groups. l’he activation energies of the p process increased in the order, hydroxyl, acetate and benzoate, indicating that the benzoate terminated polymer is most constrained.

Conclusions Whilst the generalities of molecular motion in polymer systems are widely understood, the availability of new architectures enable the effects of specific interactions and molecular distortion at a segmental level to be understood. Correlation of the data from different techniques are allowing a greater degree of precision to be introduced into the definition of the processes which are occurring and produce the opportunity of more refined theoretical models to be developed.

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IO. .

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11.

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methacrylates Macromol Rapid

l

Papers of particular Interest, published wlthin the annual period of review, have been hlghlighted as: l

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24. ..

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31.

Davis WJ, Pethrlck RA: Investigation of physical ageing in polymethylmethacrylate using positron annihilation, dielectric relaxation and dynamic mechanical thermal analysis. Polymer 1998, 39:255.

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aspects

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