Chapter 1
Overview and Prospects of Polymer Morphology

Jerold M. Schultz

Department of Chemical Engineering, University of Delaware, Newark, DE, USA

1.1 Introductory Remarks

Why are we interested in the morphology of polymers? I would like to say that our interest is in the inherent beauty and intriguing complexity of the patterns. Two examples are microphase-separated block copolymers and homopolymers crystallized from the melt. Figure 1.1 shows the range of morphologies typical to simple AB copolymers [1]. These structures are defined by the composition of the diblock copolymer: alternating plates for approximately equal amounts of components A and B, proceeding through double gyroid, rod, and sphere morphologies as the composition is made increasingly unbalanced (see Chapters 10, 14, and 15). More complicated repetitive morphologies are found in more complex block systems [1, 2]. Importantly, the repetition scales of these morphologies are of the same order as the dimensions of the molecular coils of the blocks - typically, tens of nanometers. Homopolymers crystallized from the melt display morphological features from the micrometer range to tens of nanometers. Figure 1.2 is an optical micrograph of spherulites of poly(ether ketone ketone) crystallizing from the melt. Seen in the figure are arms radiating from a central point, the arms then branching at small angles, to fill all space between the arms. Higher resolution images reveal that the arms are composed of stacks of long, ribbonlike crystals, with the molecules running in the thin direction of the crystals, as sketched in Figure 1.3 [3]. Why and how did such long-chain molecules, very highly intertangled in the melt, disentangle themselves to form this spherically symmetric array of bundles of lamellar crystals?

Figure 1.1 Morphology of AB diblock copolymers. From (a-d), in increasing composition from 0 to 50 vol%, spheres arranged on a body-centered cubic lattice, hexagonally packed cylinders, gyroid, and lamellae. Balsara and Hahn [1]. Reproduced with permission of World Scientific.

Figure 1.2 Spherulites growing into a melt of poly(ether ketone ketone) (PEKK 70/30, a copolymer of 70% terephthalate and 30% isophthalate moieties) at 280 °C.

Figure 1.3 Sketch of a growing spherulite, showing crystalline lamellae and growth arms (stacks of lamellae). Schultz [3]. Reproduced with permission of American Chemical Society.

Certainly, the beauty of and the desire to understand the complexity of systems such as block copolymers and melt-crystallized polymers have played a role in driving the study of morphology. But most of the research in this area is funded and executed because properties of the polymeric materials are tied to the morphological detail. For instance, the fine-scale repetitive morphology of block copolymers makes them well suited for photonics [4], as well as for photovoltaic [5, 6] and battery [7] applications. Figure 1.4 shows a recent result for a block copolymer used as a photovoltaic system. The block copolymer is poly(3-hexylthiophene)-block-poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2´,2?-diyl) (P3HT-b-PFTBT), with a composition of 56 wt% P3HT. With nearly equivalent volume fractions of each block, the system has an alternating plate morphology, as shown in Figure 1.4a. The I-V curve shown at the right demonstrates an efficiency of about 3%. While this efficiency is not competitive with current commercial photovoltaics, it is encouraging for the earliest stages of a new approach.

Figure 1.4 (a) Sketch of alternating lamellar morphology of a photovoltaic device made from the block copolymer poly(3-hexylthiophene)-block-poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2´,2?-diyl) (P3HT-b-PFTBT), with a composition of 56 wt% P3HT. (b) I-V curve for the device. Guo et al. [6]. Reproduced with permission of American Chemical Society.

While the study of the morphology of polymers has been an occasional topic for over a century, it became a field of study in its own right with the advent of commercial transmission electron microscopes some 60 years ago. It was only then that the fine structures unique to polymers could be resolved and directly observed. But the electron beam in electron microscopies typically destroys the specimen in a few tens of seconds, precluding much in the way of following the evolution of fine morphological detail. Studies of morphological evolution were based on less direct (but nonetheless useful) scattering, diffraction, spectroscopic, and calorimetric methods, in which local structure was educed from bulk behavior. This situation changed in the late 1990s with the advent of scanning probe microscopies and, somewhat more recently, with imaging based on spectroscopies. The current state of structural tools is detailed in Chapters 2-9 of this book.

1.2 Experimental Avenues of Morphological Research

There are broadly three avenues of investigation of morphology. One is the characterization of the morphological state of a polymeric material. All of Part 1 and parts of Part 2 of this book deal specifically with characterization. As mentioned, there would be no need for a science of polymer morphology, were the morphology unimportant in establishing properties. Morphology-property relationships are then a second important area of study. A sampling of recent morphology-property research is given in Chapters 18 and 21, with examples included in other chapters. The third avenue is the study of how processing controls morphological detail, and hence also defines the behavior of the product. A sampling of work in this area is provided in Chapters 11-16, 19, 20, and 22. The three avenues of research are treated in the following subsections.

1.2.1 Morphological Characterization: The Enabling of in situ Measurements

Because so much of this compilation is already devoted to characterization, we concentrate here on only two aspects: rapid measurements and combined techniques.

One of the most interesting developments over the few decades of morphological study has been the development of tools for following morphological development in situ during processing operations. Many of these in situ methods awaited the development of fast measurement tools. Synchrotron radiation has provided X-ray and infrared (IR) intensities orders of magnitude higher than had been possible in laboratory-scale instruments. This beam intensity, plus the creation of detectors capable of capturing an entire spectrum of data in parallel, has reduced scan times for individual measurements from the order of an hour to the order of milliseconds. The technologies that enabled such work were the development of one- [7, 8] and two-dimensional [9, 10] position-sensitive wire detectors in the 1970s and of polymer-oriented beamlines at synchrotrons, beginning in the mid-1980s [11]. In parallel, more recently, Chase and Rabolt have similarly provided a rapid advance for infrared spectroscopy, developing a parallel capture system [12-14]. Another pair of important breakthroughs in the first decade of the 2000s were the recognition that morphological development from the melt could be followed at high resolution by atomic force microscopy (AFM) [15-17], and the subsequent development of a very fast method of obtaining AFM images [18-21]. Using AFM, the same area can be probed many times at high resolution, in contrast to the situation for electron microscopies.

Another area of recent advances is in imaging using signals other than light, electrons, or neutrons. Scanning microscopies have enabled the use of any of a wide variety of signals, among which are surface friction (AFM phase mode), near-field optics, time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and infrared and Raman absorption. An example of ToF-SIMS mapping across a spherulite, from Sun et al. [22], is shown in Figure 1.5. Seen is a map of the positions from which molecular fragments representing poly(ethylene oxide) (PEO) and poly(l-lactic acid) (PLLA) occur in a ring-banded spherulite of a 50/50 blend of PEO and PLLA. This image shows a radially periodic alternation of the two components. The periodic alternation is as yet unexplained. Figure 1.6, from Cong et al. [23], shows IR images taken at three different times during the growth from the melt of an isotactic polypropylene (iPP) spherulite at 142 °C. The band at 1303 cm-1 represents crystalline iPP; the band at 998 cm-1 represents ordered sequences of iPP in the melt. The spatial disposition of the 998 cm-1 band demonstrates the preordering of iPP chains in the melt ahead of the spherulite front. IR imaging is described in detail in Chapter 7.

Figure 1.5 An ion map of a banded spherulite that formed in a 3-µm-thick film of a PLLA/PEO (50/50) blend crystallized between a silicon wafer and a Kapton cover for 5 h at 125 °C. Image obtained after removal of the Kapton. Sun et al. [22]. Reproduced with permission of Elsevier.

Figure 1.6 In situ optical microscope images of a single spherulite (set A) and the corresponding 3D images of...