Studies on structure property relationship of polyamide-6/attapulgite nanocomposites

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1 Composites Science and Technology 66 (26) COMPOSITES SCIENCE AND TECHNOLOGY Studies on structure property relationship of polyamide-6/attapulgite nanocomposites Liang Shen, Yijian Lin, Qiangguo Du *, Wei Zhong Department of Macromolecular Science, The Key Laboratory of Molecular Engineering of Polymer, Ministry of Education, People s Republic of China and Fudan University, Shanghai 2433, PR China Received 2 June 25; received in revised form 25 November 25; accepted 8 December 25 Available online 19 January 26 Abstract Polyamide-6/attapulgite nanocomposites were prepared and characterized by thermogravimetric analysis, differential scanning calorimetry, thermomechanical analysis and tensile test. From the results it can be concluded that the exhibited properties of the nanocomposites came from the intrinsic structure of the nanocomposites. The experimental results showed that the incorporation of the attapulgite silicates caused the polymer network to exhibit reinforcement in thermal and mechanical properties and slower relaxation in segments mobility compared with neat polyamides. The confinement effect was evidenced by the thermo mechanical data from dynamical mechanical analysis and thermal mechanical analysis. The nanocomposites showed dynamic modulus reinforcement, suppression of thermal expansion effect, and enhanced tensile property with increasing attapulgite loading, resulting from direct grafting of the polymer chains to the exfoliated dispersed silica fibers and the restriction on molecular mobility of polymeric segments near the silicate surface. Ó 25 Elsevier Ltd. All rights reserved. Keywords: A. Particle-reinforced composites; B. Thermomechanical properties; D. Thermogravimetric analysis; D. Differential scanning calorimetry; B. Mechanical properties 1. Introduction Polyamide-6 resin is one of the most widely used polymeric materials, which has abundant applications in aerospace, communication, engineering, and commodity industries [1]. Compared with most metal and ceramic materials, polyamide-6 resins exhibit high thermal expansion coefficient (CTE), low thermal stability, and low mechanical strength. Therefore many nano-scale inorganic materials have been introduced into polyamide-6 matrix to improve its properties, among which materials based on spherical particles such as silica [2,3], and titanium [4] and materials based on layered silicate such as montmorillonite [5 8] and mica [9,1] have been studied extensively. * Corresponding author. Tel.: ; fax: address: qgdu@fudan.edu.cn (Q. Du). These polyamide-6 based nanocomposites have demonstrated substantial improvements in overall properties, which offers scientists and engineers new challenges and opportunities to understand fundamental aspects of polymer science and to design nanocomposites with unique properties. The restricted relaxation behavior has attracted considerable attention since Jackson and McKenna [11] first reported a calorimetric study of the vitrification of glass forming liquids in controlled pore glasses. Various experimental techniques such as thermal analysis [11 15], X-ray reflectivity [16,17], dielectric relaxation [18 2], and molecular dynamics simulation [21 23] have been utilized to explore the molecular mechanism for the relaxation behavior under confined conditions. And the effects of various confining environment on the molecular dynamics of glass formers have been investigated. These environments include thin polymer surface films on substrates [24], /$ - see front matter Ó 25 Elsevier Ltd. All rights reserved. doi:1.116/j.compscitech

2 L. Shen et al. / Composites Science and Technology 66 (26) amorphous polymers confined between crystalline lamellae [25,26], crosslinking polymer networks [27,28], and nanocomposites [29,3]. Attapulgite is a family of fibrous hydrous magnesium silicates with particle size on the nanometer scale in two dimensions [31]. The structure of fibrous minerals differs from that of layered silicates used in nanocomposites in which there is a lack of continuous octahedral sheets. It has a large surface area and strong absorptive capacity that is greater than any other natural minerals [31]. In addition it had good mechanical strength and thermal stability. These properties make attapulgite an ideal candidate for reinforcing polymeric materials. In the preparation of polyamide-6/attapulgite nanocomposite, the most important step is the activation of the attapulgite fibers, which enables active organic groups (to react or graft) onto the surface of the fibers [32]. To date, very little is known about polymer/fibrous clay (such as attapulgites) systems in terms of their restricted structure and related properties. In a previous article [33], we presented a convenient, in situ polymerization route for the preparation of polyamide-6/attapulgite nanocomposites with attapulgite premodified using cetyltrimethylammonium bromide (CTAB) and toluene-2,4-diisocyanate (TDI). It reported the remarkable rheology behavior associated with the confinement effect in nanocomposites. In order to explain the confinement effect of nano-fibers on polymer segmental relaxation, we proposed a grafting-percolated model. The objective of the present paper is to clarify the relationship between the structure and the thermal and mechanical properties of our polyamide-6/attapulgite nanocomposites. 2. Experimental 2.1. Materials A series of polyamide-6 and polyamide-6/attapulgite nanocomposite samples were synthesized via in situ polymerization. Prior to that, attapulgites were modified with cetyltrimethylammonium bromide (CTAB) and toluene- 2,4-diisocyanate (TDI) according to the synthesis route described in our previous paper [33] Characterization and measurements Thermal gravimetric analysis were performed on a Pyris 1 TGA (Perkin Elmer Inc, USA), at a heating rate of 1 C min 1 under nitrogen, and the temperature range was from room temperature to 7 C; Differential scanning calorimetry was performed on a Pyris 1 DSC (Perkin Elmer Inc., USA), and all samples were subjected to the following procedure. The samples were heated to and held at 2 C for 5 min to eliminate the thermal history. They were cooled back to 5 C at1 C min 1 by liquid nitrogen, and were held at that temperature for 2 min, and finally were heated again to 2 C at 1 C min 1. All the procedures were carried out under nitrogen atmosphere; The CTEs of the polyamide-6 and polyamide-6/ attapulgite nanocomposites were measured on a NET- ZSCH TMA (Thermal mechanical analyzer) 22 cell (Netzsch Inc, Germany) at a heating rate of 5 C min 1 from 5 C to 15 C; Dynamical mechanical measurements were done on a DMA 242 (Netzsch Inc, Germany). The mode of force loading was single-cantilever. The samples 1 Wei gh t (% ) W eight ( %) 1 polyamide-6/attapulgite nanocomposites neat polyamide (%) Weight Tempe rature ( C) Fig. 1. TGA curves of PA-6; PA-6/1 wt% attapulgite; PA-6/2 wt% attapulgite; PA-6/3 wt% attapulgite; PA-6/4 wt% attapulgite; PA- 6/5 wt% attapulgite. The right inset is the amplified graph in the temperature range of 66 7 C while the left inset is the amplified graph in the temperature range of 4 48 C.

3 2244 L. Shen et al. / Composites Science and Technology 66 (26) were quickly cooled to 5 C to be equilibrated at that temperature for 5 min, then were heated to 16 C at a frequency of 1 Hz with a constant heating rate of 3 C min 1 under nitrogen atmosphere. The tensile properties of the dumbbell specimens, which were injection moulding at 25 C on a laboratory mixing molding (Atlus Inc, USA), were measured according to the standard of ISO 527/ on an Instron-5567 universal tester at room temperature in a drawing rate of 5 mm min 1 for effective sample length of 3 mm. 3. Results and discussion TGA has been used to study the thermal properties of polyamide-6/attapulgite nanocomposite samples, and the results are illustrated in Fig. 1. It is generally believed that the introduction of inorganic components into organic materials can improve their thermal stability according to the observation that these species usually have good thermal stability. From TGA curves in Fig 1 it can be seen that the decomposition temperature of the nanocomposites (which is determined by the position of the maximum on Table 1 DSC characteristic parameters of polyamide-6 and polyamide-6/attapulgite nanocomposites Sample T m ( C) DH m (J g 1 ) T mc ( C) DH mc (J g 1 ) T m T mc ( C) the differential TGA curve), T d, increases with the amount of added attapulgites. The weight of the residue at 7 Cis regarded as the real silicate content. Wrapping of the attapulgite fibers may have shielded the polyamide chains from the chemical and thermal attacks. Table 1 listed the melting point (T m ), melting enthalpy (DH m ), crystallization temperature during melt cooling (T mc ), crystallization enthalpy (DH mc ) and the supercooling degree (T m T mc ) obtained from DSC analyses of the samples. The DSC curves (Fig. 2) show that the melting point of the nanocomposites, T m, increased with the attapulgites loading after the thermal history is erased. The parameter T mc is used to evaluate the nucleation during polymer melt crystallization. The higher T mc is, the easier it is to forms nuclei via regular arrangement of polymer segments, i.e., higher crystallizing ability. Since stable nuclei are difficult to form near melting point, the supercooling degree is the thermodynamic drive, and the smaller (T m T mc ) is the higher the crystallizing ability indicated [3]. As shown in Table 1, with the addition of attapulgite silicate, the tendency for polyamide-6 to crystallize increases. These results imply that attapulgite in polyamide-6 may play a role as a nucleating agent for the crystallization and for the decrease of the supercooling temperature of the crystallization. In general, the dynamic mechanical analysis of polyamide-6 displays an a peak in the tand curve that is originated from the movement of the longer molecular chains in the amorphous region, corresponding to the glass transition temperature (T g ). Fig 3 is the DMA results of the nanocomposites prepared through in situ polymerization. The T g of the polyamide-6/attapulgite nanocomposites shifts to higher temperature as silicates loading increases, End o increasing Attapulgite loading Fig. 2. DSC curves of PA-6; PA-6/1 wt% attapulgite; PA-6/2 wt% attapulgite; PA-6/3 wt% attapulgite; PA-6/4 wt% attapulgite; PA-6/ 5 wt% attapulgite.

4 L. Shen et al. / Composites Science and Technology 66 (26) Tan d E ( M Pa ) 1 8 Increasing Attapulgite loading Tem perature ( C) Fig. 3. Dynamic mechanical spectra of DMA curves of PA-6; PA-6/1 wt% attapulgite; PA-6/2 wt% attapulgite; PA-6/3 wt% attapulgite; PA-6/4 wt% attapulgite; PA-6/5 wt% attapulgite. which indicates that good adhesion between the modified attapulgite fibers and the polyamide-6 matrix can limit the motion of the polyamide-6 molecular chains. The storage modulus E of the samples is plotted against temperature in Fig. 3. The E value decreases with increasing temperature, and increases with attapulgite content. It is clear that, over the entire temperature range, the E values of the nanocomposites are much higher than that of the neat polyamide-6, which is in accordance with the observation that those nanocomposites have higher thermal dimensional stabilities than the neat polymer. The incorporation of attapulgite silicate fibers into polymer chains, due to the high mass and volume of fibers, slows down the segmental motion. In segmental motion the silicate fibers requires a void volume to move into when it changes location. It is difficult for attapulgite fibers to move through a volume of more or less entangled polyamide-6 chains. The larger mass also resists rapid translocation. Attaching an attapulgite fiber to a polymer chain segment is somewhat like attaching an anchor to a jump rope. Even at high temperature (>T g ), the motion of segments with attapulgite bound by TDI will experience restrictions. This could account for the higher T g and E values of the nanocomposites over the entire temperature range. The direct attachment of attapulgite fibers onto the bulky polyaimde-6 macromolecules makes polymer chains more rigid. Their segmental movements are significantly restricted and cannot even happen at low temperatures. The higher T g and

5 2246 L. Shen et al. / Composites Science and Technology 66 (26) T m of the nanocomposites, in comparison to that of the pure polyamide-6, should be due to its more rigid distributed silicate fibers. The thermal mechanical performance of the neat polyamide-6 and nanocomposites containing 1, 2, 3, 4, and 5 wt% modified attapulgite fibers is measured by TMA under an external compression of approximately 8 cn, and the results are shown in Fig. 4. According to DMA results, the glass transition temperatures of all the samples were around 5 C, above which TMA curves diverged distinctly. As the attapulgite fiber content increases, the deformation above T g is suppressed, therefore the thermal dimensional stability of the nanocomposites at high temperature is improved. The increase in T g of the nanocomposites suggests that restricted molecular segmental motions occur at the interface between the attapulgite fibers and the polyamide-6 as the results of the formation of charge-transfer complexes. When segmental motions of polyamide-6 s long chains are restricted by the modified attapulgite fibers through physical and chemical interactions, the T g is expected to increase. The thermal expansion coefficient (CTE) of the samples also decreased from C 1 in the pure polyamide-6 to 12, 93, and C 1 for samples with attapulgite fibers loading of 1, 3, and 5 wt%, respectively. It is understandable that restriction on polymer chain rearrangement exerted by the silicate fibers dispersed in the nanocomposites enhances the moduli of the nanocomposites but reduces the elongations at break. As the concentration of the modified attapulgite fibers increases in the polyamide-6 matrix, the tensile modulus and the strength increase while the elongation at break decreases. The detailed mechanical property data are listed in Table 2, in which each data point is obtained by averaging the results of five test samples. When the attapulgite silicate concentration reach to 5 wt%, the compressed samples exhibited a significant modulus improvement up to 1.88 GPa, while the tensile modulus was 57.1 MPa and the elongation at break decreased to 39.5%. From our analysis of the experimental results, we propose the domain relaxation model that is presented in Fig. 5(B). The slower relaxation characteristic (domain in grey) corresponds to the motion of segments located in the interphase of the silicate fibers, and the retardation stemmed from the neighboring segment is shielded by the individual silicate fiber. The faster relaxation mode (domain in polymer bulk) corresponds to the normal segmental motion of polyamide-6 network, therefore no retardation and shielding effects are anticipated in this mode. This proposed domain relaxation model provides a general understanding of the restricted relaxation that occurs in fibrous silicate-polymer nanocomposites. As the content of exfoliation increases, more polymer chains diffuse into Table 2 Tensile properties of polyamide-6 and polyamide-6/silica composites Sample Elongation at break (%) Tensile strength (MPa) 1% Young s modulus (MPa) 22.1 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 58 5 dl(µ m) Fig. 4. Thermal mechanical analysis (TMA) results of the neat polyamide-6 and nanocomposites: PA-6; PA-6/1 wt% attapulgite; PA-6/2 wt% attapulgite; PA-6/3 wt% attapulgite; PA-6/4 wt% attapulgite; PA-6/5 wt% attapulgite.

6 L. Shen et al. / Composites Science and Technology 66 (26) Fig. 5. SEM morphology (A) and Domain relaxation model (B) depicting the restricted relaxation of the exfoliated silicate fiber-polymer nanocomposites. the silicate interfibrillar free spaces, resulting in greater cooperativity between intercalated segments. Thus, as the interfibrillar spacing increases, the faster relaxation is diminishes. For the case of higher attapulgite loading, the faster relaxation mode is further hindered, even eliminated. Based on the SEM morphology (Fig. 5A) and other experimental results in our previous work [33], we clarified the properties of our nanocomposites in terms of the grafting-percolated fully exfoliated structure. As concluded in Ref. [33] that the polyamide-6 chains grafted onto the solid attapulgite fibers, the restriction effect is expected. Relaxation spectrum in reference [33] also showed that the faster relaxation mode shifted to slower relaxation mode with increasing attapulgite loading. 4. Conclusions Polyamide-6/attapulgite nanocomposites consisting of individual fibrous silicate have been prepared. The increase in T d and T g of our nanocomposites could be due to the good interfacial adhesion between the polyamide-6 matrix and silicate fibers or to the restriction of molecular mobility of polymeric segments near the silicate surface. The nanocomposites also exhibit dynamic modulus reinforcement, suppressed the thermal expand effects, and enhanced tensile property with adequate filler loading, which leads to their new application in aerospace, communication, engineering, and commodity industries. Based on our experiments and analysis, we concluded that a system with fully exfoliated silicate fiber dispersion and strong interactions (polyamide-6 chains grafted onto the attapulgite surfaces bridged by toluene-2,4-diisocyanate) has slow relaxation behavior and confinement effect. This present work has established a visible picture for the molecular motion involved in polymer nanocomposites, which contributes to our understanding of the structure-properties relationship. References [1] Cheremisinoff, Nicholas P. Handbook of engineering polymeric materials. New York: Marcel Dekker Inc.; [2] Garcia M, Garcia-Turiel J, Norder B, Chavez F, Kooi BJ, van Zyl WE, et al. Polyamide-6/silica nanocomposites. Adv Eng Mater 24;6(9): [3] Shen L, Du QG, Wang HT, Zhong W, Yang YL. In situ polymerization and characterization of polyamide-6/silica nanocomposites derived from water glass. Polym Int 24;53(8): [4] Zhu MF, Xing Q, He HK, Zhang Y, Chen YM, Potschke P, et al. Preparation of PA6/nano titanium dioxide (TiO2) composites and their spinnability. Macromol Sym 24;21: [5] Wu SS, Jiang DJ, Ouyang XD, Wu F, Shen J. The structure and properties of PA6/MMT nanocomposites prepared by melt compounding. Polym Eng Sci 24;44(11):27 4. [6] Tjong SC, Bao SP. Preparation and nonisothermal crystallization behavior of Polyamide 6/montmorillonite. J Polym Sci Polym Phys 24;42(15): [7] Hu Y, Wang SF, Ling ZH, Zhuang YL, Chen ZY, Fan WC. Preparation and combustion properties of flame retardant nylon 6/ montmorillonite nanocomposite. Macromol Mater Eng 23; 288: [8] Nour MA. Polymer/clay nanocomposites. Polimery 22; 47: [9] Wang TW, Zhang J, Shao YG. Reinforcement mica to polyamide-6. China Plast 22;16(8):4 3. [1] Messersmith PB, Giannelis EP. Synthesis and characterization of layered silicate-epoxy nanocomposites. Chem Mater 1994; 6: [11] Jackson CL, Mckenna GB. The glass-transition of organic liquids confined to small pores. J Non-Cryst Solids 1991;131: [12] Vaia RA, Sauer BB, Tse OK, Giannelis EP. Relaxations of confined chains in polymer nanocomposites: Glass transition properties of poly (ethylene oxide) intercalated in montmorillonite. J Polym Sci: Polym Phys 1997;35: [13] Jackson CL, Mckenna GB. Vitrification and crystallization of organic liquids confined to nanoscale pores. Chem Mater 1996;8(8): [14] Schonhoff M, Larsson A, Welzel PB, Kuckling D. Thermoreversible polymers adsorbed to colloidal silica: A H-1 NMR and DSC study of the phase transition in confined geometry. J Phys Chem B 22;16(32):78 8. [15] Schonhals A, Goering H, Schick C. Segmental and chain dynamics of polymers: From the bulk to the confined state. J Non-Cryst Solids 22;35(1 3):14 9. [16] Wallace WE, van Zanten JH, Wu WL. Influence of an impenetrable interface on a polymer glass-transition temperature. Phys Rev E 1995;52:R [17] Tsui OKC, Russell TP, Hawker CJ. Effect of interfacial interactions on the glass transition of polymer thin films. Macromolecules 21;34(16): [18] Anastasiadis SH, Karatasos K, Vlachos G. Nanoscopic-confinement effects on local dynamics. Phys Rev Lett 2;84:915 8.

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