Surface Modification of Nanoclay for the Synthesis of Polycaprolactone (PCL) – Clay Nanocomposite

This paper presents a new modification method to modify the surface of nanoclay (NaMMT) to increase its d-spacing using Aminopropylisooctyl Polyhedral Oligomeric Silsesquioxane (AP-POSS) and the fabrication of Polycaprolactone (PCL) nanocomposite through solution intercalation technique. The structure and morphology of pure nanoclay, modified nanoclay (POSSMMT) and the PCL nanocomposite were characterized by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Field Emission Scanning Electron Microscopy (FESEM). XRD revealed that the d-spacing of the POSS-MMT is increased by 0.64 nm as compared to pure nanoclay. FTIR and FESEM results also showed that AP-POSS were well dispersed and intercalated throughout the interlayer space of Na-MMT. An exfoliated structure was also observed for PCL/POSS-MMT nanocomposite. Thermal properties of the nanocomposite were investigated using Thermal Gravimetry Analysis (TGA) which recorded highest degradation temperature for PCL/POSS-MMT 1% nanocomposite which is 394.1 ̊C at 50% weight loss (T50%) but a decrease in degradation temperature when POSS-MMT content is increased and Differential Scanning Calorimetry (DSC) analysis which showed highest melting and crystallization temperature for PCL/POSS-MMT 5% nanocomposite which is 56.6 ̊C and 32.7 ̊C respectively whereas a decrease in degree of crystallinity for PCL/POSS-MMT nanocomposite as compared to PCL/Na-MMT nanocomposite. This study affords an efficient modification method to obtain organoclay with larger interlayer d-spacing to enhance the properties of polymer nanocomposite.


Introduction
Plastics are ideal for many applications such as in packaging, building materials and commodities but it can lead to waste disposal problems.Accumulation of plastic at the end of its life cycle had increased drastically on the earth causing serious pollution problems.Therefore, this motivated many researchers to conduct studies to produce a biodegradable and environmental friendly polymer nanocomposite to produce plastics.There has been a strong emphasis in the development of polymer nanocomposites for the last 20 years.Polymer nanocomposites consist of inorganic nanofiller and organic polymers represent a new class of materials that exhibit improved performance compared to their microcomposite counterparts [1].
Polycaprolactone (PCL) is considered to be a good biodegradable polymer because of the low production cost and easy processibility in large scale production.PCL have good commercial potential for plastics but their low thermal and mechanical properties for further processing restrict their use in a wide range of applications [2].Therefore, these properties can be improved through the preparation of PCL nanocomposite by incorporating nanofiller such as nanoclay.
Sodium montmorillonite (Na-MMT) is the most abundant nanoclay mineral that is easily available with a good swelling capacity, high cation exchange capacity and high surface area [3].However, the hydrophilic nature of nanoclay hinders homogenous dispersion of nanoclay in the hydrophobic polymer matrix [4].Modifying the nanoclay surface through organic treatment will give a hydrophobic environment to the galleries of the Na-MMT and enhance its compatibility with the polymer matrix.Conventional modification method is the ion-exchange reaction using cationic salts such as alkyl ammonium or phosphonium salts.There are some drawbacks in this modification process such as low thermal stability of the cationic salts and the compounds are less readily intercalated in the polymer melts.Alkyl ammonium salt will decompose at temperature above 170-180˚C which shows that it is not suitable for the high temperature melt processing techniques [5].
Thus, a new modification method will be the main aim of this study.The surfactant method by using Aminopropylisooctyl Polyhedral Oligomeric Silsesquioxane (AP-POSS) is developed to give a better surface modification for the Na-MMT.The main MATEC Web of Conferences 150, 02005 (2018) https://doi.org/10.1051/matecconf/201815002005MUCET 2017 purpose of the surface modification of Na-MMT is to expand the interlayer d-spacing of the nanoclay gallery so that polymer molecules can penetrate as well as to enhance its miscibility with polymer to achieve a good dispersion.AP-POSS has a high thermal stability which is up to 300˚C.Therefore, an enhanced PCL nanocomposite can be produced with modified nanoclay.
Polymer nanocomposites usually can be prepared by using three different techniques such as in-situ polymerisation, melt blending and solution intercalation.Solution intercalation method is considered in this study because only a small amount of polymer nanocomposite is to be fabricated in a laboratory scale whereas the other two techniques is preferred for industrial scale.Thus, the main aim of this study is to develop a new modification method for dispersing nanofillers in PCL nanocomposite and to fabricate PCL nanocomposite through solution intercalation technique.

Modification of Na-MMT using Surfactant Method
Five (5) g of Na-MMT was suspended in 250 ml of deionised water at 25˚C with stirring for 2 hours.2.332 g of AP-POSS was dissolved in 6 ml of ethanol and acidified with 4 ml of acetic acid.It is then added dropwise to the Na-MMT suspension over a period of 20 min [6].The mixed suspension was then continuously stirred at 70˚C for 24 hours to obtain the organoclay (POSS-MMT).Then the organoclay was harvested by centrifuge, dried in oven for 48 hours and ground to pass mesh sieve.

Fabrication of PCL Nanocomposite
The nanoclay (1wt%, 3wt% and 5wt%) was stirred in chloroform for 1 hours.PCL (1g) was also stirred with chloroform (20ml) to get a solution of PCL/Chloroform.That solution was then dropped into nanoclay/chloroform suspension and stirred for 4 hours.The suspension was poured into a clean petri dish and left covered for 2 days.After 2 days, the film was dried in a vacuum oven for 24 hours at 40˚C to remove the residual traces of chloroform.

Fabrication of PCL Nanocomposite
XRD data was recorded on a MiniFlex Rigaku using graphite-filtered Cu-Kα radiation (1.25, 0.154 nm).The diffractometer was controlled using Diffrac Plus XRD Commander and the raw data was manipulated using EVA software [6].All the data were collected between the ranges of 3˚ to 80˚ 2θ with a resolution of 0.02˚.The d-spacing of the nanoclay were calculated by using the Bragg's law.

Sin θ = nλ /2d
(1) Where, d is the spacing between the layers of the nanoclay, λ is the wavelength of the X-Ray which is 0.154 nm, θ is the angle at the maximum point of the first peak in the spectra, n is the order of diffraction which 1.
FTIR was recorded by using Nicolet Omnic 3 FTIR spectrophotometer.The samples were used in the solid form.The background of the samples was in KBr standard.After the background is completed, the reading for the samples was collected.
FESEM analysis was done by using the JEOL FESEM JSM-7100F.'Smart FESEM User Interface' program was run.The samples were mounted on the universal holder with stub forceps.The focus knob was turned to sharp the scanned images.FESEM images then were saved.

Thermal Analysis of PCL nanocomposite
TGA analysis was carried out by using Perkin Elmer TGA Q500.The samples were heated from 25˚C to 600˚C at a heating rate of 5˚C/min under nitrogen atmosphere.A mass change versus temperature curve was obtained.
DSC analysis was done using Perkin Elmer DSC Q1000 under nitrogen atmosphere.Measurement was performed from room temperature to 160˚C at heating rate of 10˚C/min.The samples were stand stilled for 5 minutes to erase any previous thermal history.Subsequent cooling and heating cycles was recorded respectively.The degree of crystallinity ( cr) of the sample was calculated by using equation shown below.
ΔH100 represents heat of fusion of 100% crystalline PCL [136.1 J/g [7]], ΔHf represents experimental heat of fusion and WPCL represents weight fraction of PCL.All these important characteristic bands are situated in each PCL nanocomposite structure and show the presence of PCL.A new characteristic peak can be observed at the region of 3470-3485 cm -1 in the entire PCL nanocomposites.These peaks represent the broad band of stretching vibration of the interlayer water in the Na-MMT silicate.This band shows a slight increase in its intensity correlated with the increasing nanoclay content in each nanocomposite samples [14].The most intense characteristic bands for Na-MMT are located in the region 1050-990 cm -1 attests the Si-O stretching vibrations.Unfortunately, those characteristic bands cannot be distinguished because PCL has multiple bands in the same IR spectral range [14].The presence of important principal characteristic bands of POSS-MMT falls in the region between 3000 cm -1 to 2800 cm -1 .These bands which represent the CH2 stretching vibration and the N-H bonds are also present in the pure PCL structure.It also can be seen a small increase in the intensity of that particular peaks with increasing POSS-MMT content.This result clearly verifies that intercalation of PCL with both unmodified and modified nanoclay has occurred.

Fig. 5. FTIR Spectra of Pure PCL and PCL Nanocomposite
The FESEM analysis for PCL nanocomposite shows that POSS-treated nanoclay nanocomposite gives a matrix with well distributed particles compared to pure nanoclay nanocomposite as shown in Figure 6.In PCL/Na-MMT nanocomposite, nanoclay tends to stack together and do not disperse uniformly in the PCL matrices as shown in Figure 6(a).The Na-MMT tends to be agglomerated and stacked on the PCL matrix.This is because the immiscibility between Na-MMT and PCL is a major issue due to poor interfacial adhesion [15].The distribution of modified nanoclay layers in PCL/POSS-MMT nanocomposite appeared more uniform throughout the polymer matrix.The difference in morphology of PCL/Na-MMT 5% and PCL/POSS-MMT 5% nanocomposite is very obvious where the agglomerated particles disappear completely in PCL/POSS-MMT 5% nanocomposite as shown in Thermal degradation of pure PCL and PCL nanocomposite was monitored through TGA.PCL materials degrade with a large peak in single step as observed for each sample in Figure 7.The degradation temperatures for 50% weight loss (T50%) decreases gradually from 392.9˚C for pure PCL to 391.4˚C for PCL/Na-MMT 5% nanocomposite and this is due to the incompatibility of Na-MMT with PCL matrix together with its hydrophilic character which can be related to the decomposition of physically adsorbed water and water molecules surrounding the exchangeable sites in Na-MMT.A noticeable increase in degradation temperature at T50% can be observed for PCL/POSS-MMT 1% nanocomposite which is 394.1˚C.The shift to higher degradation temperature is because POSS-MMT may act as a mass transport barrier to the volatile product that is generated during decomposition or diffusion hindrance of the decomposited volatiles through nanocomposite [16].In a study conducted previously, the degradation temperature was higher for PCL/MMT-C8H17 nanocomposite and lower for PCL/Na-MMT nanocomposites compared to neat PCL at T50% [16].PCL/POSS-MMT 3% and 5% nanocomposite have the lowest degradation temperature among other samples which is 382.3˚C and 364.8˚C respectively.This can be associated with two major effects as the content of POSS-MMT increased.Firstly, the POSS-MMT could have functioned as an inorganic catalyst for polymer degradation [17].Secondly, the AP-POSS surfactant can be decomposed at lower temperatures following the Hoffman elimination where these resulting degradation products of AP-POSS could have possibly contributed in catalyzing the degradation of PCL matrix [9].The resistance to thermal degradation was improved when the organically modified nanoclay content is 1 wt% in PCL matrix but the effect levels off independently of the modified nanoclay beyond that content [16].Thus, the results obtained becomes obvious that the modified nanoclay may possess two opposing functions in thermal stability of polymer nanocomposites where it improves the thermal stability by barrier effect and decreases the thermal stability by catalytic effect on the degradation of the polymer matrix.The effect of nanoclay on the melting temperature (Tm), crystallization temperature (Tc) and degree of crystallinity (Xcr) of PCL was analysed by DSC.Based on Table 1, it can be clearly seen that PCL/POSS-MMT nanocomposites possess a higher Tm compared to PCL/Na-MMT nanocomposite loaded with equal amount of nanoclay and pure PCL where it shows 56.6˚C for PCL/POSS-MMT 5% nanocomposite.In the case of Tc, the PCL/Na-MMT nanocomposite experiences a decrease in the Tc from pure PCL to PCL/Na-MMT 5% nanocomposite which is from 31.9˚C to 31.0˚C then increases when POSS-MMT is incorporated into the PCL which is up to 32.7˚C for PCL/POSS-MMT 5% nanocomposite.Similar results were reported by Di et al. (2003) [18], where Tc of the polymer matrix increased with the addition of a small amount of C30B, an organoclay to the PCL matrix.The increase in both these temperatures are because of increased order of lamellar crystals of PCL or the molecular mobility of PCL segments was suppressed by spatial confinement due to the presence of organoclay particles [19].There also could be nano-reinforcement effect of Na-MMT layers together with the intercalating AP-POSS surfactant which were well dispersed in the PCL matrix.The Tm and Tc of nanocomposite depend on the degree of dispersion of nanoclay and its dispersed sizes in the polymer matrix.The PCL/Na-MMT nanocomposites have a higher Xcr (calculated by equation 2) compared to the PCL/POSS-MMT nanocomposite with the same amount of nanoclay loading.The presence of surfactant at the interface can prevent the nucleation process and leads to decrease in the crystallinity [20].The decrease in Xcr is also due to the reduced mobility of the polymer chains in presence of strongly interacting nanoclay [18].Thus, incorporation of modified nanoclay into PCL matrix improves the Tm and Tc compared to unmodified nanoclay but reduces the Xcr.

Fig. 4 .
Fig. 4. XRD Pattern for POSS-MMT and PCL/POSS-MMT Nanocomposite The FTIR spectra for pure PCL and PCL nanocomposite samples are shown in Figure 5.The pure PCL have peaks located at 2930-2945 cm -1 representing CH2 bonds stretching vibration.The vibration of C=O bonds present at 1724 cm -1 band.The region of 1275 cm -1 to 1050 cm -1 indicates the C-O-C aliphatic ether stretching vibrations.All these important characteristic bands are situated in each PCL nanocomposite structure and show the presence of PCL.A new characteristic peak can be observed at the region of 3470-3485 cm -1 in the entire PCL nanocomposites.These peaks represent the broad band of stretching vibration of the interlayer water in the Na-MMT silicate.This band shows a slight increase in its intensity correlated with the increasing nanoclay content in each nanocomposite samples[14].The most intense characteristic bands for Na-MMT are located in the region 1050-990 cm -1 attests the Si-O stretching vibrations.Unfortunately, those characteristic bands cannot be distinguished because PCL has multiple bands in the same IR spectral range[14].The presence of important principal characteristic bands of POSS-MMT falls in the region between 3000 cm -1 to 2800 cm -1 .These bands which represent the CH2 stretching vibration and the N-H bonds are also present in the pure

Table 1 .
DSC Data for Pure PCL and PCL NanocompositeThe new AP-POSS based surfactant method for modification of nanoclay was done successfully where a significant increase in the interlayer d-spacing from 0.593 nm in Na-MMT to 1.233 nm in POSS-MMT has been achieved.The AP-POSS surfactant was successfully intercalated into the interlayer galleries of Na-MMT.The disappearance of diffraction peak in XRD for PCL/POSS-MMT nanocomposite and FESEM images verifies a good dispersion of POSS-MMT layers in the PCL matrix.TGA results showed highest degradation temperature of 394.1˚C at T50% for PCL/POSS-MMT 1% nanocomposite.DSC data showed highest Tm of 56.6˚C and Tc of 32.7˚C for PCL/POSS-MMT 5% nanocomposite with a lower Xcr of 35.8% as compared to PCL/Na-MMT nanocomposite.PCL/POSS-MMT nanocomposite has enhanced thermal properties compared to pure PCL.Organic modification of nanoclay with new surfactant method gives good potential for incorporation with various biodegradable polymers such as Polybutylene Succinate and Polylactic Acid to give enhanced properties.