Characterization of Crystalline Polymers by Raman Spectroscopy and

22 Downloaded by KTH ROYAL INST OF TECHNOLOGY on October 30, 2015 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch022

Characterization of Crystalline Polymers by Raman Spectroscopy and Differential Scanning Calorimetry Leo Mandelkern Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306

Semicrystalline polymers formed by flexible chain molecules display complex structures and morphology. A set of independent variables governed by the molecular constitution and the crystallization conditions has been identified. These variables control microscopic as well as macroscopic properties. To understand the broad range of properties of such crystalline systems, a wide variety of experimental techniques has to be used to measure these structural variables and to comprehend the wide range in values that these systems are known to assume. Two such techniques are differential scanning calorimetry and Raman spectroscopy. In this chapter we illustrate, by a selected set of examples, how these two techniques can be used to establish melting temperatures of thermodynamic significance, to quantitatively describe the phase structure, and to elucidate the nature of the interlamellar regions of semicrystalline polymers.

POLYMERS C A N C R Y S T A L L I Z E I N A R E A S O N A B L E T I M E only at temperatures that are well below their melting points. In addition, the crystallization process is rarely, if ever, complete. Hence, a nonequilibrium state develops. Consequently, when a polymer system crystallizes from the pure melt, a morphologically complex polycrystalline state is formed. Both the macroscopic and microscopic properties of this semicrystalline state will depend on the structural and morphological features that define the state. Therefore, the key independent structural variables that describe the crystalline state

0065-2393/90/0227-0377$06.(X)/0 © 1990 American Chemical Society

In Polymer Characterization; Craver, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1990.

378

POLYMER CHARACTERIZATION

must be identified and described. A set of independent structural variables, previously identified (I), either individually or in particular combinations contribute to and control a specific property. The variables that have been identified so far in this connection are the level, or degree, of crystallinity; the structure of the residual noncrystalline region; the crystallite thickness distribution; the structure and relative amount of the interfacial region; the crystallite structure; the supermolecular structure; and the melting tem-

Downloaded by KTH ROYAL INST OF TECHNOLOGY on October 30, 2015 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch022

perature. Because the transformation from the liquid to the crystalline state is not complete, the system can be characterized by the degree of crystallinity or extent of transformation that exists under given conditions. Understanding the detailed nature of the phase structure that has developed thus becomes a very important matter. After the initial discovery of the lamellarlike crystallite habit, the view was expounded that the concept of the degree of crystallinity was invalid. Instead, the system was proposed (2-5) to consist of a crystalline matrix in which defects were randomly interspersed. More modern theoretical and experimental studies have banished this concept to obscurity. As will be shown, the concept of the degree of crystallinity can be placed on a rigorous and quantitative basis. The noncrystalline portions of a system comprise two major regions. The interfacial region is associated with the basal plane of the lamellar crystallites, and the interlamellar region reflects that portion of the system where the chain units connect crystallites. It is now recognized that the interfacial region is not made up of regularly folded chains with an occasional, rare deviation from this regularity. Nor does this region result from the nucleation of regular folded chains that grow to mature crystallites of the same form (6). The problem and structural details that are primarily concerned with the interfacial structure involve the distribution of units that return to the crystallite of origin and the amount and extent of this region. Quantitatively defining this structure is very important because it defines many properties. Theoretical advances have, however, recently been made in resolving this problem (7-11).

In the interlamellar region, the chain units that connect

the crystallites are in random conformation so that the region is isotropic. Within the general concept of isotropy, however, a variety of detailed structures can exist. These structures need to be quantitatively specified in the future. Although lamellar crystallites are the typical form for homopolymers over a wide range of molecular weights (12-15) as well as for random copolymers of surprisingly high co-unit content (16), other factors of crystallite structure are of importance. These include the chain tilt, the curvature of the lamellae, and their lateral extent. The supermolecular structure represents the arrangement of the lamellar crystallites into some type of higher organization. Spherulites, for example, are one type of supermolecular structure. The nature of the superstructure that forms depends on many factors,


22.

MANDELKERN

Raman Spectroscopy and DSC of Crystalline

Polymers

379


i n c l u d i n g t h e m o l e c u l a r w e i g h t , t h e crystallization t e m p e r a t u r e , a n d t h e c h a i n s t r u c t u r e (17-21). O n e i m p o r t a n t c o n c l u s i o n that has r e s u l t e d from studies o f t h e s u p e r m o l e c u l a r structure u n d e r c o n t r o l l e d conditions is that spherulites are n o t a u n i v e r s a l m o d e o f p o l y m e r crystallization. T h i s p o i n t has b e e n d e m o n s t r a t e d from studies o f l i n e a r p o l y e t h y l e n e (17), b r a n c h e d p o l y e t h y l e n e (18,19), p o l y e t h y l e n e c o p o l y m e r s (20), p o l y e t h y l e n e oxide (21), a n d isotactic p o l y p r o p y l e n e (22, 23). T h e crystallite thickness d i s t r i b u t i o n involves t h e d e t e r m i n a t i o n o f the crystallite size i n t h e c h a i n d i r e c t i o n . Because o f the w i d e v a r i e t y o f structural variables that are p e r t i n e n t to the p r o b l e m , m a n y different e x p e r i m e n t a l techniques are n e e d e d to u n derstand properties. T h i s c h a p t e r w i l l focus attention o n t w o s u c h t e c h n i q u e s , n a m e l y R a m a n spectroscopy a n d differential scanning c a l o r i m e t r y . T h e examples t a k e n to illustrate various aspects o f t h e p r o b l e m w i l l b e from t h e p o l y e t h y l e n e s . T h e s e p o l y m e r s , w i t h t h e i r w i d e range o f m o l e c u l a r weights a n d structures, serve as v e r y good models for crystalline systems of flexible chains. D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y ( D S C ) measurements give t h e m e l t i n g t e m p e r a t u r e , t h e e n t h a l p y o f fusion, w h i c h leads to t h e degree o f c r y s t a l l i n i t y , t h e glass t e m p e r a t u r e , a n d , o f course, t h e specific heat. T h e uses o f R a m a n spectroscopy for p a r t i c u l a r applications are as follows: • I n t e r n a l m o d e s for phase structure (24-26) • L o n g i t u d i n a l acoustical m o d e ( L A M ) for crystallite thickness d i s t r i b u t i o n (27-34) • D - L A M for long-range conformation d i s o r d e r (35-38) T h e i n t e r n a l m o d e s , w h i c h give us t h e phase structure, l i e i n t h e r e g i o n o f a p p r o x i m a t e l y 9 0 0 - 1 5 0 0 c m " . T h e L A M (longitudinal acoustical mode), w h i c h gives us the crystallite thickness d i s t r i b u t i o n , lies i n the range of about 5 - 2 0 c m " . T h e D - L A M , w h i c h gives us a measure o f the long-range c o n formational d i s o r d e r , is i n t h e range of 200 c m " . 1

1

1

W e shall a p p l y these t w o t e c h n i q u e s to several selected examples. M a n y m o r e examples c o u l d b e discussed e v e n w h e n r e s t r i c t i n g t h e discussion to j u s t these t w o methods. R a m a n spectroscopy a n d differential scanning c a l o r i m e t r y represent o n l y a p o r t i o n o f the e x p e r i m e n t a l techniques r e q u i r e d to c o m p l e t e l y describe t h e p r o p e r t i e s a n d structure o f semicrystalline p o l y mers.

Discussion Melting Temperature.

T h e first example of these techniques is the

d e t e r m i n a t i o n o f the m e l t i n g t e m p e r a t u r e (39), w h i c h is v e r y often a straightf o r w a r d process: d e t e r m i n i n g t h e e n d o t h e r m i c peak i n t h e D S C . H o w e v e r ,


380


many cases present the problem of melting-recrystallization. Very often this phenomenon is not detected, so that the differential scan gives a very misleading melting temperature that results in major problems in a detailed analysis. A n example of this problem is given in Figure 1, which is a thermogram for a dilute solution formed crystal of polyethylene (39). From the peak in the endotherm, the melting temperature is expected to be close to 130 ° C . However, in this case, there is a strong suspicion that this value is


too high because the melting endotherm is found to be independent of crystallite thickness. Hence, a melting-recrystallization process is probably 40).

occurring (39,

i

100

NO

1

1

120

130

r

140

T °C Figure 1. DSC thermogram for dilute solution crystallized polyethylene (M = 1.66 x 10 ). Conditions: heating rate, 5 K/min; range, 2 mcalls; and sample mass, 0.72 mg. (Reproduced with permission from reference 39. Copyright 1986 John Wiley ir Sons.) w

5

To ascertain if this conjecture is correct, in that we are not observing the melting point characteristic of the initial sample, we studied the distribution of crystallite thicknesses by monitoring the Raman L A M as a function of temperature. Figure 2 gives a normalized plot of the ordered sequence lengths against the length as derived from the spectra (39). For the initial sample, and for those heated to 112 ° C , the size distribution stays constant, is very narrow, and is centered at about 130 A . As the annealing temperature is raised to higher temperatures, 120.5 ° C and above, major changes are observed in the L A M , as is reflected in the curves given in Figure 2. After heating at 120.5 ° C , a bimodal crystallite thickness distribution develops.



22.

MANDELKERN

Raman Spectroscopy and DSC of Crystalline

Polymers

381

L (Angstrom) Figure 2. Normalized plot of number of ordered sequences, K N , of length L against L as derived from the spectra of sample illustrated in Figure 1 . Key: —, original sample; • • •, T = 120.5; - - , T = 123.5; T = 125; -• • •-, T = 150 ° C . ( T is the annealing temperature.) (Reproduced with permission from reference 39. Copyright 1986 John Wiley it Sons.) L

A

A

A

A

A

One population corresponds to the initial crystallite size distribution, the other to crystallites that were formed from the pure melt. These results lend themselves to the straightforward interpretation that partial melting and recrystallization from the melt has taken place. After the sample is annealed at 123.5 °C, the resulting size distribution indicates that almost all of the sample has melted and recrystallized under this condition. Only a very small concentration of the original crystallites remains. After the sample is annealed at 125 °C, the size distribution is essentially the same as after crystallization from the pure melt, that is, from 150 °C. Therefore, the melting of the original crystallites is complete at 125 °C, and the fusion process is well under way at 120.5 °C. The melting temperature of this sample is in the restricted range of 123.5-125 °C and is not at 130 °C, as might have been interpreted from the initial thermogram. We must therefore be careful that the thermogram does indeed represent the melting of the crystallites that are initially formed if we are interested in melting temperatures of thermodynamic significance.

Phase Structure.

The phase structures of semicrystalline polymers

are of primary importance in understanding their properties. Three distinct regions are involved: 1. the ordered crystalline region, which, for polyethylene, represents the orthorhombic unit cell structure 2. the liquidlike, disordered region having an isotropic structure


382


3. the interfacial region, which involves chain units that connect these two different structural regions Thus, a chain can traverse all three regions or be restricted to the crystalline and interfacial ones. Flory pointed out in 1949 (41) that a sharp demarcation between the ordered crystalline region and the random liquidlike region cannot in general take place because of the spatial requirements of chains


emerging from the crystallite. This concept has been theoretically developed in more detail (7-11,

42).

Several different kinds of measurements can be used to describe the phase structure:

Definition

Type of

x).

degree of crystallinity

density

x) „


enthalpy of fusion


Raman internal

degree of liquidlike

Raman internal

Descriptive

a (i -

Variable

A

Measurement

modes

Characterization of Crystalline Polymers by Raman Spectroscopy and

Recommend Documents