Chapter 1
Virgin and Recycled Polymers Applied to Advanced Nanocomposites

Luis Claudio Mendes* and Sibele Piedade Cestari

Instituto de Macromoléculas Professora Eloisa Mano - IMA, Universidade Federal do Rio de Janeiro - UFRJ, Rio de Janeiro, Brazil

*Corresponding author: lcmendes@ima.ufrj.br

Abstract

The study and development of nanostructured polymers is an expanding field. New strategies on advanced polymeric nanocomposites and hybrid materials have been created, to be used in different areas. Nanocomposites can improve characteristics of virgin and recycled polymers; they can also resemble biomaterials for medical or drug delivery applications. We studied some neat and modified materials that are seldom used as filler in nanocomposites - zinc oxide and zirconium phosphate - and added it to recycled polymers matrices - polycarbonate and poly(ethylene terephthalate). The use of nanoscaled fillers in polymer composites can improve properties like morphology, resistance to ultraviolet radiation, mechanical performance, crystallinity, and molecular mobility. Advanced nanocomposites can actually improve the effectiveness, sustainability, and performance of materials.

Keywords: Polymers, nanocomposites, advanced materials, recycling, sustainability

1.1 Introduction

Definitely, nanoscience and nanotechnology entered our lives in order to bring benefits for society. In particular, researches on polymeric nanocomposites intend to create solutions for daily problems. Polymeric nanocomposites can be considered advanced and sustainable composites. These materials are expanding. Several new strategies for developing advanced polymeric nanocomposites and hybrid (nanocomposites/microcomposites) have been created, in order to be used in many different areas (Thakur et al., 2012a,b; 2014a,b). Due to the high aspect ratio of the disperse phase, the properties are improved with low filler content. The disperse phase may resemble leaves - nanolayers - where only one dimension is in nanoscale; have the shape of nanotubes - two dimensions are in the nanometer range; and finally be a nanoparticle - three dimensions are on the nanometric scale. In polymeric nanocomposites, polymer - virgin, recycled, and renewable - is the phase that allows the incorporation and takes advantage of the properties which the nanosize substances can offer. Both academia and industry understand the importance of polymeric nanocomposites in the current state of society development.

Due to the similarities with the mineral constituents of bone tissue, enamel, and dentin of teeth, hydroxyapatite (HA) is an important class of biomaterial (Sato, Hotta et al., 2006; Fomin, Barinov et al., 2009; Brundavanam, Jiang et al., 2011). Besides immune response, other qualities - osteoinduction, osteoconductive, and osteointegration - indicate HA for using in medical or drug delivery devices. Biomimetic, hydrothermal, sol-gel, and precipitation processes have been studied as routes for producing collagen/HA composites for bone and dental repairs (Hilson, 1986; Orlovskii, Komlev et al., 2002; Ficai, Andronescu et al., 2010; Zhang, Tang et al., 2010). In order to prepare a collagen/HA nanocomposite as osteoinductive of the pulp-dentin complex, we investigated the influence of the presence of collagen (COLL) on structural and morphological characteristics (Mendes, Ribeiro et al., 2012).

Thermogravimetric (TG) analysis (Figures 1.1 and 1.2) of the materials showed that COLL has two stages of degradation. The first one (25-200 °C, 8%) was ascribed to the loss of water and the second one (270-500 °C, 65%) to the polymer chain degradation. The HA without collagen showed only one stage of degradation (150-235 °C) ascribed to the loss of water. In contrast, the HA synthesized with COLL showed two stages of degradation. The initial stage was similar to that of HA without collagen, and a second stage arose at higher temperatures (425-450 °C). We concluded that some chemical and/or physical interactions between components have happened, increasing the thermal stability of COLL (Sionkowska & Kozlowska, 2010).

Figure 1.1 TG curves of COLL, HA, and HA/COLL. (Reproduced with permission from L.C. Mendes, G.L. Ribeiro, and R.C. Marques, Materials Sciences and Applications, Materials Sciences & Applications, 2012, 3, 8. ©2012, Scientific Research.)

Figure 1.2 DTG curves of COLL, HA, and HA/COLL. (Reproduced with permission from L.C. Mendes, G.L. Ribeiro, and R.C. Marques, Materials Sciences and Applications, Materials Sciences & Applications, 2012, 3, 8. ©2012, Scientific Research.)

Scanning electron microscope (SEM) images (Figure 1.3) of the materials showed morphological differences between the two HAs. The HA synthesized in the presence of COLL showed morphology similar to regular microrods, while in the absence of COLL an irregular cluster was obtained. Ca/P ratios of 1.89 and 2.38 were calculated for HA without and with COLL, respectively. The addition of COLL influenced the HA chain growth, and different chain catenation was proposed as shown in Figure 1.4.

Figure 1.3 SEM photomicrographs of HA (a), COLL (b), and HA/COLL (c). (Reproduced with permission from L.C. Mendes, G.L. Ribeiro, and R.C. Marques, Materials Sciences and Applications, Materials Sciences & Applications, 2012, 3, 8. ©2012, Scientific Research.)

Figure 1.4 Schematic representation of the feasible repeat unit of the HA: Ca/P = 1.89 and 2.38, without and with COLL, respectively. (Reproduced with permission from L.C. Mendes, G.L. Ribeiro, and R.C. Marques, Materials Sciences and Applications, Materials Sciences & Applications, 2012, 3, 8. ©2012, Scientific Research.)

Polycarbonate is a very important engineering polymer largely used in outdoor exposure. In order to improve its resistance against ultraviolet radiation and increase its mechanical performance, nanocomposites of recycled polycarbonate (rPC) and nano-zinc oxide (nZnO) were studied in a master's dissertation (Carvalho, 2015). The effects of nZnO, gamma radiation, and UV light exposure were assessed. The action of UV light was monitored by variation of carbonyl group along the rPC chains, using carbonyl index (CI). Samples without UV exposition (Table 1.1) showed decrease of CI. Both nZnO nanoparticles and gamma radiation fostered a certain degree of degradation on the rPC chains.

Table 1.1 CI of the materials before UV light exposure (Carvalho, 2015).

(a)Percentage loss (in parenthesis)

The effect of UV light on the CI was surprising (Table 1.2). After 50 h, the CI decreased for all selected materials. The percentage loss (in parenthesis) followed this order: filled rPC (2% nZnO), neat rPC, irradiated rPC (30 kGy), and filled and irradiated rPC (2% nZnO and 30 kGy). After 100 h, there was a recovery of the CI. The percentage loss was lesser according to this order: filled rPC (2% nZnO), neat rPC, filled and irradiated rPC (2% nZnO and 30 kGy), and irradiated rPC (30 kGy). Thérias et al. (Collin, Bussière et al., 2012) suggested that the occurrence of cross-linking at longer exposure periods, based on the decrease of the absorption at 1186 cm-1 (isopropylidene group) in their investigation of UV photoaging on PC. We assumed that the peroxide radicals produced from the reaction of rPC free radicals with oxygen have been activated by UV radiation. The activated species could have reacted with end groups of the rPC oligomeric chains (recombination) and/or with hydroxyl groups onto the nanoparticles surface through esterification reaction (Figure 1.5). These reactions could have fostered the increase of CI during the UV light exposure.

Table 1.2 rPC CI: (a) neat rPC, (b) irradiated (30 kGy), (c) filled (2% nZnO), and (d) filled and irradiated (2% nZnO and 30 kGy), after UV light exposure (Carvalho, 2015).